for this kind of things, the forums are still alive

i dont know about any application to do this in maemo (except zoutube but its youtube only)

what i know is a workaround: do PLAY with your flash player (megavideo, vimeo, etc) and while is buffering, the flv is stored in ~/MyDocs/tmp/Flash12345 where 12345 is a random number.

so you can let start buffering and watch it with mplayer-cli, copy it to your MyDocs or wathever you want, its your flv now.

it gets deleted when you close the window, but copy or move it elsewere and its safe.