Hi again. I forgot that you are using IR not RF. So I assume you are genereating a 38khz carrier. I'm not actually sure if IR receivers are sussceptible to IR noise because the requirement for a 38khz carrier should elliminate most noise issues if not all, I would guess. I would just put a logic probe on your serin pin or scope it. Sorry for the RF information. Here is a link to an Energizer (cylindrical) battery performance information, if you plan to use alkalines. http://data.energizer.com/PDFs/alkaline_appman.pdf. There's a lot more out there for other battery types if you search for "battery discharge curves" for example.

I found that my 16F876 operated down to about 2.4 volts with the brownout detect disabled. As such I am running my transmitter on 2 AAs. Of course there is low voltage versions of most PICs including the 16F628A as I recall.