Ok I recently had an issue with gmail not syncing, I was getting notified of new emails but the app was refusing to load any new email, tho it showed that there was new messages for me.
A quick browse to Settings / Data Synchronization / Google showed a red exclamation mark with the message ‘Sync is experiencing probles, it’ll be back soon’ I left it for 6hrs and it didn’t improve.
I resolved the problem by going to Settings / Applications / Manage Applications / Gmail and clicked ‘clear data’
Open Gmail, pres Menu, then refresh Gmail starts to resync and all was good in the world. 😀
Hope this helps some ppl