I am running it without any problems and I have been hacking the hell out of mine so far.
Not to pick on you personally, but that's the last thing you should be doing. It's already a beta where the point is to test the current code base for any problems. Unless you're adding exclusively plugin code (i.e., not touching the vBulletin code itself), it defeats the purpose of a beta program to modify your installation.
And as a side note nitpick, vBulletin versioning works in a major.minor.revision format, which means the current beta version is 3.5.0, not 3.50 which is entirely different (equates to 3.50.0).