Lifehack recently had an article demonstrating
Parallel's Coherence view which allows for Windows XP/Vista to run on the same screen as Mac OSX, making for some pretty cross platform craziness. I personally tried it out, and it does work really well.

However, what I found out by moving any Windows application window is that you can easily see the windows background behind the application window. As such, Parallels doesn't really MERGE both OSes together, but rather have Windows and Mac layered over each other, and so in order to move or use Windows you NEED to touch a windows app to move the layer up, and vice versa for Mac. Not a full merger, but an interesting setup nevertheless.

One huge disadvantage I can see using Parallels in comparison to Boot Camp is that Parallels is, for all intents and purpose, a virtual machine, and would not be able to push hardware natively like that under Boot Camp. DirectX will NOT run under Parallel's Windows VM, so unfortunately for all you Mac gamers out there you will not be able to play and heavy duty games that require DirectX of any sort under Parallels Windows. However, if you're using it as a strict work-related machine it'd satisfy all your needs.