They're probably using main or extended profile H.264, which the N900 doesn't support for a hardware decode. If you're on windows, I'd recommend mediainfo to check this. On Linux you can just use the 'file' command.

Mplayer can decode those profiles, in theory, but it's not hardware-accelerated & the CPU won't be able to keep up (hence the sync issues). It's not a practical solution for any reasonably-sized video clip.

Like paulkoan says, either transcode (aim for iphone/ipod profiles) or watch it on another device.