I am having an issue as well with my 770. A couple of weeks ago, I uploaded a theme (Aqua), and suddenly, I couldn't boot from either the flash or the mmc. It starts to boot, and then goes back to the Boot Menu.

I attempted to re-flash, and I started with a test load:

sudo ~/bin/flasher-3.0 -l -b -k os2006-linux-2.6.16-39-1+ipv6+ext2+mmcplus52Mhz-zImage

and what I got back was:

Version of 'sw-release': <no version>
Sending kernel image (1357 kB)...
100% (1357 of 1357 kB, avg. 658 kB/s)
Sending request 0x82 failed: Cannot send after transport endpoint shutdown

Any ideas on what I can do to correct this problem?