I have the exact same problem and I hope this forum will hep me resolve it since I have tried without success over the last week.
I have a rooted HTC desire with MIUI GB Rom. Everything worked fine till a week back the gmail app suddenly stopped syncing and has been since then stuck on the "waiting to sync" page - "Your email will appear shortly". When i manually sync it, the app force closes. Even if i go into "accounts and sync" and uncheck and re-check auto-sync, the app force closes.
Googling this issue showed that tons of other folks have had this issue but I haven't found a solution. I've literally tried everything (i think):
Manual sync, uncheck and recheck auto sync, phone reboot, removing battery, uninstalling updates, uninstalling the app itself via Titanium backup and reinstalling via market, clearing cache and data of gmail app, clearing cache+dalvik from recovery, full wipe and reinstall of rom, factory reset and trying different rom, upgrading to Nikez lente ICS rom...
The issue happens only with my account and not other accounts i sync.
I just moved ALL my inbox mail to a new unsynced label but it did not help. I deleted a lot of my mail and also changed IMAP settings to sync nothing but the inbox which is now empty...but no success.
EDIT: Oh and the proposed solution of deleting data/cache has not helped either