If you have an existing copy of vbulletin, you should order for the presale.
http://www.vbulletin.com/forum/showthread.php?t=320055
http://www.vbulletin.com/
There is discussion that they are changing the licensing method and rumors that the license terms will all be reset on Nov 1 when 4.0 comes out. Based on what I am reading, it appears that a purchase of 4.0 gives you upgrades for the 4.x series for life. Meaning that the annual subscription is gone. When 5.0 is released, you would have the option of upgrading there.
In any case, if you have VB 3.x, you should take a look. If you don't have it, you will want to look at 4.0's 'social cms frontend' + the forums.