Didn't have time to read all comments so please disregard if already been posted.
Youtube would not work for me either. Poking around I went to the wap browser (t-mobile) instead, searched for youtube, clicked on the m.youtube.com (for mobile), AND IT WORKED FROM THERE.
For the newbie, the wap browser is the globe with the pink T on your bb's desktop.
Hope this helps.
Add: I am on wifi and I did click the box on APN authentication enabled: options->advanced->tcp/ip
Me too ladylis. I've tried every fix that people suggest. Changing the apn, setting browser to hotspot, refreshing w javascript enabled, using m.youtube (duuhhr; these ppl are not qualified to give advice)... Nothin works. Now here's the annoying part: everything works perfect at my parents' house and my brother's house... Just not my house. UMA works everywhere. Internet works everywhere. M.youtube works everywhere... Just not over my (premium) comcast wifi. So... Wtf?
use vuclip web if you haven't already it works great