I can view YouTube on my blackberry, they sense my blackberry and redirect to http://m.youtube.com/ , where all the videos are in 3GP format, which plays on my blackberry media player.
I went to dslreports.com and ran the speed test on my Samsung Blackjack on AT&T’s 3G network. The result was 671 kbit/sec.
When my wife gets home, if I can pry it away from her, I’ll try it on her iPhone.
These guys did speed tests on the 3 big companies in various places around the country
The Definitive Coast-to-Coast 3G Data Test
Results can vary a lot even within a given city metro area.