Sega's currently holding a contest for Sonic's 19th birthday (which was last month), giving out prices to the 100 best entries, best entry winning a trip to Tokio to meet Team Sonic.
The catch is, they've actually let through that they're not very satisfied with the entries they got so far, so far as to say that anything better than "show my game collection" or "musicvideo using game-cutscenes" has a valid chance of winning. [link]
The contest ends Jule 12th, here's some rules and suggestions on what to do: [link]
Well, they didn't really mention anything about flash, but they also didn't say that it was not allowed.
Personally, I don't have much of an idea, but perhaps by sharing this info I'm giving a chance to some other people. Eh, I might update this later if I should get any ideas to ask you about what you think of them.