I just found this link:
http://www.afsa.org.za/Portals/0/Documents/Welding%20corner/pocket-guide.pdf
which seems pretty good on how to weld aluminium
Also see www.millerwelds.com for loads of useful stuff on welding.

Regards
Hugh

[Edited on 16/6/09 by hughpinder]

Excellent! Brilliant linked. Saved onto my desktop.

I've got a reel of ali filler and some argon in the garage, at some point I really need to get practising. I only need to tack weld some bits together to hold them together before filling / brazing with technoweld.... (it's for a supercharger airbox- I doubt MIGd ali would take positive pressure- not my welding anyway!!)...

Great find, thanks for sharing!