You CAN stream videos over the internet. I do.
Blackberry Media player will work with some formats (you can search the forums for more on this). I know that some folks have talked about ORB (which is what I use) working. I stream recorded video and TV quite successfully.
However, youtube is not something I have used, although it seems to me that there is a client available somewhere on the forums.
With BB Browser, when a video stream is detected, it will generally open the BB Media player automatically, and stream if the file type is right.