yes I am using the predownloaded images. I use them all the time here, it'd be crazy to be downloading the same 1.5gb updates all the time
I also noticed my downloaded 10.7.3 combo's certificate expired on the 23rd, haven't seen that before. tho I have seen it when the computer's clock is set wrong. I assume if I downloaded it again now it would have a new signature on it. (that had nothing to do with the problem)
I have reinstalled 10.7.0 on that one and it now accepts the 10.7.3 update.
On a few occasions I've seen a worse variation on the problem, where the base installer also refuses to run for apparently similar reasons. For THOSE, I have to set aside the system files and do a clean reinstall and import stuff, big hassle. So I'd really like to know what it's looking at.