Its veryvery difficult to di. I had made a hack that did this on my site but I removed it when I upgraded to vB3.
The biggest difficulty is with urls. if you want to 'auto' link the word bob and someone types in
www.bob.com then it is going to screw up your link.
The other problem is that it is VERY processor hungry. The more auto links you have (or variations on the theme)... the more it will slow things down.
I guess I could see if I could find my old code for you though...