There are many companies in the US that do it, here is one...
http://weldcraftwheels.com/index-1.html
There is a company in Germany too...
http://www.rindt-felgen.com/verbreiteru ... elgen.html
But ive come across a company here in the uk.
http://www.bigjimracing.co.uk/home.html
I spoke to the guy and he tells me he's been doing it for years with no comebacks or problems, the conversation was to take my 7j BBS mahles to 8.5j by cutting and banding them. he also tells me That older BBS's are perfect to widen as they are made from a better quality alloy.
Here's a pic of a pair the German company have done.

here's the thread relating to these ^
http://forums.bimmerforums.com/forum/sh ... ht=widened
What are your thoughts on this?