This has been an ongoing question since Bootcamp was released. I've actually been searching the answer myself. I can understand why Bootcamp must be installed on an internal drive, mainly because USB 2.0 simply isn't fast enough to run efficiently from an external hdd, in my opinion. However, with the introduction of USB 3.0 and Thunderbolt, I don't see why this isn't an option anymore.
Personally, I don't use Windows that much, mainly for gaming. In these instances, I would much rather connect my MBP to an external hdd and boot to that for windows rather than taking up precious ssd space for windows. My understanding is that with Thunderbolt, the speed is supposed to be akin to having a drive interally connected (just like my eSATA adapter for my early 2008 MBP; and in this case, Thunderbolt is even faster than that).
Hopefully, the question will either be answered by the community or Apple themselves soon.
Here is some literature I have found, none really answering the question entirely:
It is Windows not Apple, not Boot Camp, and you can have two SSDs, while 256GB SSD have dropped to under $200 (Crucial m4 sale Memory day was $180).
Booting Windows from external devices is not supported. You can run a VM instead of natively though.
And you can put all your data on externals and save space on internal drive - for both Mac and Windows.
you can always put OS X and boot external IF the interface and controller are supported of course, not totally different there, but doable.
A laptop will always be limited in what you can and cannot do or having two internal SSDs or other setups.
Even PCs are pretty much the same.
Does Apple have USB3 or SATA3 external ports that are bootable for OS X? then connect your SSD to those externally and use internal drive for Windows. OS X can get along fine for OS and apps on small 60GB and even smaller probably.