Then the XPS Studio 16 might be a better alternative than the Alienware on the Windows side. The Alienware would be a real boat anchor. Both the XPS 16 and the Macbook are in the <7lb range, and this is really the maximum that most normal people could (slightly un)comfortably carry in the course of regular travels.
The 17-inch Macbook Pro has decent all-round specs. I can't really make any truly valid personal comment on it as I don't have one yet (will be a couple of weeks since I've only just ordered them) but I have to say as an owner of the smaller Macbook Pro and the aluminium Macbooks, while the 'unibody' concept is a looker and it feels premium, it doesn't result in the strongest structure out there - and this will be doubly true for the 17-inch (the main reason the battery isn't replaceable is that making it s would have weakened the chassis further). And while previous Macbook Pros were prone to denting, these are more prone to bending (and in the case of the Macbook Air's I've had, actually have to be bent back into shape infrequently) - which is a potentially more serious issue.
The XPS 16 - and I can actually speak now from personal experience as I have one - is more traditionally constructed with a magnesium inner frame, and is certainly more likely to tolerate a heavier beating than a 'unibody' machine. It's a capable performer especially if you load it out and it'll still be notably cheaper than a Macbook Pro, while pretty much matching it in terms of general performance (yet to be confirmed personally, but the core components including the graphics chips are virtually identical in terms of performance).
2.66 17-inch MBP, 4Gb, 320Gb 7200 HDD, iLife, Final Cut Express, Microsoft Office H&S and 3-year return-to-Store Applecare = $3697.
2.66 XPS Studio 16, 4Gb, 500Gb 7200 HDD, Sony Imagination Studio (direct equivalent to iLife + Final Cut Express), Microsoft Office H&S and 3-year onsite support with accidental damage and theft protection = $2515.
That's a thousand bucks you can put towards e.g. the full-blown copy of Sony Vegas. The XPS 16 screen is also 16:9 - so you can view and edit HD video natively.
If you want longevity away from a power socket, the Macbook Pro definitely has an edge in that it can switch between integrated and discrete graphics - and while my experience with Apple's claims vs real runtime on my Macs leads me to believe that the 17-inch definitely doesn't have an 8-hour endurance (Air - claimed, 5 hours: real: 3.75 on extreme power saving modes with a barely visibly dimmed screen - the percentage of this disparity is pretty typical for all Apple notebooks) - still, a 6-hour potential power-saving endurance with the integrated low-power graphics switched in is not bad at all. The non-switchable-ATI-graphics Dell - with the heavier 9-cell battery - delivers about 3.75 hours of power-saving-mode runtime which is definitely worse, but it's not bad in absolutes and you can also buy additional batteries to swap - and if you do intend to work in the field, the ability to carry several batteries with you may be crucial.
In terms of gaming, the Alienware clearly has the edge over both the Dell and the Apple, both of which will perform similarly... but are you willing to tolerate the ~*4lb* increase in your portage?