I replaced the battery. Man, it's a tough sucker to find. My local Fry's Electronics did not have it, although they usually have all sorts of batteries. After one dive lasting just short of an hour, the warning did not appear. I'm going to test out the battery and see if it really has failed. Would a multimeter from Snap-On or Mac Tools work on a Lithium Ion battery? The battery that I removed from the transmitter is made by Varta. If the problem persists, I will have my LDS take care of it. At least, it's better to have a problem with the transmitter than with the wrist unit. I can still dive my back-up SPG.
@Hatul:
TNRonin also noted his transmitter uses a CR 2/3AA battery. When I purchased my Luna, the LDS did inform me that Scubparo had made a design change to the transmitter. Unfortunately, Scubapro has not updated the owner's manual, which does state it erroneously uses the CR2450 batteries (should make errata and state transmitter with xxxx production date have CR 2/3AA.