I've had a problem on my HTC Desire over the last 3 days where I couldn't log into to GTalk, and my Google Calendar wouldn't sync with my PC.
I'd also tried to send a couple of emails with gmail, and that hadn't worked either, although my normal non google mail was working perfectly.
I had had no low memory warnings although lately sometimes apps would not install unless I installed them one at a time.
Thinking it might be a low memory problem, even though I appeared to have 13 Mb free, I started deleting unwanted apps.
Suddenly, things started working again after a few deletions.
Therefore, it must have been caused by some sort of low memory log jam.