All of the new Alienware Desktops use a proprietary power supply.  Meaning there is one very large modular plug that all the wires lead out of so that if you need to replace or upgrade the power supply with another model from Alienware, you can just unplug the harness without having ro reqire the entire case.  There is also a contact point built on to the side of the power supply to power the lighting on the left panel that I beleive you would not be able to incorporate into an aftermarket PSU. 

That being said, you can probably replace the PSU if you rewire the entire computer as well as long as it has the proper type and number of connectors.  I say 'probably' because I'm not 100% sure that there isn't anyting else non-standard about the power supply.  I sent my Aurora back (died after 2 weeks) and have re-ordered an Aurora ALX so I didn't spend enough time with it to answer 100% on this.  I am pretty sure you would lose the lighting on the side panel though with an aftermarket PSU.

I thought I read somewhere the the PSU was a user replaceable part, non standard? I haven't really checked to be honest, thankfully I went with the 875w option heh

I remember in the promo videos from the Tokyo Game Show and on Gamespot they touted the power supplies as 'easily upgradeable' and the are as long as you're upgrading with another Alienware power supply.  A bit misleading, if you ask me.

I thought I read somewhere the the PSU was a user replaceable part, non standard? I haven't really checked to be honest, thankfully I went with the 875w option heh