As far as I'm concerned, Flash 10 is a bit of a kludge at the moment.

I'm having all sorts of issues when viewing Youtube and other Flash intensive sites. Youtube only plays about 70% of the video and doesn't download the thumbnails for the videos half the time. Facebook has been a pain to use since the update, and that's just 2 sites on my PC.

As for the N900, I have the same issues as you over 3G and Wi-Fi.
If I download a 720p mp4 video via Firefox PC and copy it over to the N900, it plays like a charm.