I uninstalled iTunes 10.7 and re-installed it (after trying a lot of the other things Apple recommends with no success). That seems to have backed up my iPhone OK, and iPad is chugging along now. Hmm, got an popup error message about session, although it seems to be "transferring my purchases." And completed without displaying an error indication, just the battery state indicator.
Tried again. "iTunes could not sync information to the iPad ... because a connection could not be established to the iPad. (Which is sitting there unlocked with a wired connection and happily talking to iTunes otherwise.)
Tried again. Deleted some apps in iTunes not on iPad that had updates available, downloaded the updates to the remaining one. Sync worked. So maybe the problem is around apps updated on iPad and NOT updated in the PC Apps collection, or something like that. Programmer expected updates to be downloaded into iTunes then to iPad perhaps?
I'm leaving the WiFi synch turned off and trying wired only, to see if that helps (per some of the other online speculation re iTunes not getting app info from iPad nor session to iPhone).
BTW, the only relevant changes on my PC in several months were Windows security updates (and Norton signatures in background).