I'm glad I got you interested in the C40 combo.
Jonathan is right.
The C4040 is more expensive than the C40. However the casing is the other way around. But the difference of the cam and housing is very minimal. The PT-012 goes down to 40m whilst the PT-010 is rated at 30m. I've heard mixed responses when PT-010 goes beyond 30m - some unscaffed, some hairline cracks. Me, I'm just around 20-25m, no worries.
The PT-012 uses double stainless steel latches for the door. The PT-010 uses a single plastic "arm". I feel a lot safer with the stainless steel latch. I think they should use it for all their housing. But I've never heard flooding problems due to this "my stupid" latch problem. Mostly o-rings and dirt - so don't be concerned at all.
One thing I don't like about the PT-012 is the metal ring around the glass port. It is painted, hence the paint chips - even before I dive it's already scratched.
One other cosmetic thing, I don't know why they picked "that" color for the housing. I hate it - the gals love it.
I used to own the C3030 with the PT-005 housing. The one main reason for the switch was SIZE.
Now for the differences between C4040 and C40 (There are several threads regarding this in the Olympus forum on dpreview- you might wanna checkit out).
C4040: f1.8 - f2.6 lens
C40 : f2.8 - 4.8 lens
C4040: Dial switch with 5 modes
C40 : Dial switch with 10 modes (including my favourites)
The 10 modes makes it easy for the novice when taking ground photos. Just pick a mode e.g. landscape, night scene, portrait, etc. The "my favourite" setting is my favourite. It lets you store settings accordingly (init. zoom, a/s/m mode, white bal., etc) and uses them when I pick the my favourite setting on the dial.
I find this feature very useful for underwater photography. I set "my fav" to f8 and 1/500s at widest zoom, manual mode, normal flash, macro, auto white bal. and spot metering. When taking macros underwater, I'll use "my fav". When taking normal shots e.g. my dive buddy, I'll use the A/S/M for these kinda shots where I push up the flash 2 notches and accordingly, set the aperture or shutter priority as desired. The white balance is manually set to my dive slate.
The reason I did this bcos shots other than macros w/o using a strobe usually need more flash power to fill in the colors. The manual white balance helps a little as well.
C4040: Add-on lens/filter capability
C40 : Nope - Too bad. (so interchangeable lens..not a chance) But I figured that I'll probably purchase the C2100 or C700 for extra zoom fitted with my desired filters. That way, I'll have the best of both worlds. Afterall I carry my C40 most of the time. (you never know when that shot of a lifetime comes about). I wouldnt lug around a C2100 - not even a C3030.
However there are wide and macro lens for the housing for both the C4040 and C40. These attach to the camera housing - not the cam. Don't know how they perform top side.
C4040: min macro distance = 20cm (acoording to steve's digicam)
C40 : min macro distance = 10cm
C4040: 4 batteries
C40 : 2 batteries - but note that the C40 dries up the batts faster. This is not a concern for ground shots but not so underwater. So I change freshly charged batts before each dive and use the lcd sparingly. Set to sleep in 30sec. Otherwise, I use the lithium batts. They can really last. I've tried 3 dives continuous - yet to dry up.
C4040: lens cap
C40 : sliding cover (trivial, but I hate lens caps -always dropping them)
C4040: No built-in speaker
C40 : Built-in speaker (so you can hear the sound from your movies, but you can always hear them on a hooked up tv or computer)
C4040: One infra-red sensor
C40 : Two infra-red sensor - 1 front, 1 rear. But frankly, the rear sensor is pathetic. I am almost 1-2cm close before the IR works.
C4040: AE Lock
C40 : None
C4040: Zoom 3x
C40 : Zoom 2.8x (I can't really tell the diff)
C4040: LCD 1.8"
C440 : LCD 1.5" (The 1.5" LCD has a higher resolution - it in fact looks better than the 1.8")
C4040: External Flash hook up capability
C40 : None. For underwater shots, a slave strobe using fiber optic cables does the job. No flooding problems with connecting cables.
C4040: Viewfinder with diopter control adjuster
C40 : Normal Viewfinder
The obvious difference is of course size wise. The C4040 has an extra Multi-Spot metering feature. I don't know anything abt it. You might like to check that. The flash range is slightly different. I've tested the flash side by side - can't seem to see a big diff.
I did not use any strobe except the built-in flash (I cant afford a strobe yet). You are pretty much limited to macros. Anything else usually come out greenish or blueish. I am saving up for a strobe and a macro add-on lens, at the same time brushing up my buoyancy skills. I'm a very new diver with only 10 dives.
I had the dillema of choosing between the C4040 and the C40. I'm glad I took the C40 choice bcos the size is just so sweet. But this is personal preference.
Newhamster is right abt the S40. Don't forgo it just bcos nobody seems to talk abt it. I'm sure its just as great a cam. I didn't pick S40 bcos I had a few SM cards, so dont wanna switch to CF. I also got used to Olympus stuff and menus. The S40 feels larger and heavier. Again just my own preferences. Go to a shop and try them all out. Remember, you'll probably be using the cam 90% top side and 10% down under. Maybe you could try to prioritize the features and your requirements.
My god have I rattled. But I just wanna hopefully spare you the agonies I had when I went researching resources for the ultimate decision.
I hope I helped.