I, too, experience this issue. The main culprits are c, h, and k for some reason. I'd much rather Palm turn this "feature" off or at least let me choose if I want it on or not.
I also posted this in forums.precentral.net/palm-pre/242390-major-keyboard-problem-test-out.html. Sorry about the text links, my post count apparently isn't high enough to post the real thing.
Here's what I found on the Palm support forums: forums.palm.com/t5/webOS-Hardware/difficulty-typing-spacebar-doesn-t-register-after-comma-key/td-p/230692. The last post is about 10 months ago. I've asked for an update on the issue. Perhaps if enough of us with the comma-space problem post, we can get a satisfying resolution.
Another option: would it be possible to write a patch that fixes the issue or eliminates "fat finger" prevention altogether?