Flashearth is a very heavy site that strains even a decent desktop computer or laptop. It will play Youtube and other reasonable flash content fine if you have wifi or a decent 3G connection.

Don't expect the N900 or any other pocket-size device to be able to play stuff like Flashearth or Hulu. You'll have to wait for phones with dual-core processors and 1G+ RAM for that kind of capability. Those are a year, maybe 2 years away.