I had the same problems on my Incredible: Gmail app that won't sync, Market that won't open, GoogleTalk that opens and closes faster'n a whore's door on a Craigslist coupon day. I finally figured it out.
Today, I had downloaded upgrades for GMail, Market and Pandora. I installed all three and that's when everything went to hell.
Since then, I removed all three updates in Applications>Manage>clicked each one>Uninstall Update, and now everything works.
Google/HTC, please don't tell us it's a software issue from downloading erroneous apps. I've downloaded six apps: Pandora, Wells Fargo, Docs to Go, Photoshop Express Mobile, Doubletwist and Advanced Task Killer. The phone should clearly be able to handle that. This is obviously an OS issue.
Please resolve it.
Iphone and Blackberry committed the egregious error of blaming the users for bad phone technology (Apple with that silly $30 rubber band and Blackberry with a $100 OS fix so you can sync to Macs). That's why I bought a Droid.