I've come across the 300MB issue before but on another phone i'm seeing 7.06GB of usage under other and nothing has been put on it music/video or photos which i checked first. I ran the patches for the 300MB thinking it maybe the same but still have the same issue.

Phone was an unlocked 1.0.2 that was virginised and jailbreakme.com/anysim method for 1.1.1

Also tried deleting the media folder and rebooting.