Yep, my 3G iPhone also streams youtube videos over 3G effortlessly, however over WiFi its s-s-s-slooooow. It's not my Router - fact. I own a Desktop PC, Laptop, PS3 and Wii - my Girlfriend also owns an HTC touch diamond - Not one of these devices has a problem streaming youtube videos - but on the iPhone? Struggles to loads videos, then plays them for about 5 seconds before stalling again..
To all those reading these words on your iPhone over WiFi, head over to the BBC iPlayer to see how video streaming can and should be done..