Wirelessly posted (BlackBerry 8700g: BlackBerry8700/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100)
I have had issues with read statuses and deletes (at least from GMail to the BlackBerry) with GMail. Yahoo! seems to be more of a true two-way sync, or at least it is faster. And the read statuses are updated as well.