Personally I think it's a huge waste of money.
I have a 23" ACD, which while great is horribly limited by the fact that it uses DVI only. The only possible connector I could find was a VGA adapter for the 360, but even then, you've got a lot of connection conversion to do, and if you don't like clunk which is probably why you're shallow enough to run it on the ACD, you probably don't want a billion converters running from your MB to your ACD.
Also keep in mind the ACD is at 1920x1200, which is above the standard resolution of a 1080p TV. So in other words, even if you get it onto your monitor, there's going to be back scaling of the resolutions to match the odd number of pixels and aspect ratio difference between standard HDTV resolutions and the resolution on the ACD - in other words, your picture will look a little strange as pixels get scaled according to the larger resolution of your monitor.
Best bet, IMO, is to do something like this:
1) Get the USB TV tuner from El Gato - runs about $150 IIRC.
2) Connect your X360 to the tuner, then to your computer.
3) Run the EyeTV software and have your MB scale up the resolution to your ACD.
4) Run your X360 and see how it is.
You should also keep in mind, however, that if you run THROUGH your MB to broadcast the image data, its the MB that's processing all those frames at a giant resolution. If you've read some of the issues with HDTV viewing on Macbooks currently, you'll notice that it takes up a LARGE portion of your CPU power, especially in the MBs which have no dedicated video card to bolster some of the processing. However, this is a more cost effective solution to your shallow problem.
My best advice is get the Dell and be happy.
Cheers.