You can use any strobe with the D100. It just won't support TTL. Nikon digital SLR's use a new sensor. They refer to this as D-TTL. It is proprietary and won't support existing TTL. As a matter of fact, you must use a non-TTL sync cord, which Ikelite makes.
Yes, Aquatica and Seacam make housings for Nikon speedlights. The speedlight must be a DX version to do D-TTL. You can only use one though, so no dual strobe setup. The speedlight is weak for underwater and the color temperature isn't perfect either, but of course with digital, you can correct it fairly easily. The speedlight option works for macro, but overall just using existing strobes in manual is preferred by most.
The S2 uses the existing Nikon TTL sensor and is therefore capable of running any TTL strobe in TTL mode. The color is considered more vibrant in the S2, also.
Although it would seem that the S2 is a better choice, there are other factors. Ikelite is the only manufacturer that has anounced its intent to house the S2. It will be in an MD size case, which is quite bulky. Housings for the D100 be available from Ikelite, Sea and Sea, Aquatica, Seacam, and Amphibico?
Nikon Capture 3 software allows for easy manipulation of RAW images. Exposure can be changed +/- two stops and white balance adjusted after the shot has been taken. This lessens the pain of no TTL.
Ikelite and Sea and Sea are supposed to have their housings ready in September. Aquatica hopes to have prototype at DEMA in late October. Seacam hopes to have prototype at DEMA and production in the spring of 2003. Remember, these announcements are often like vaporware.