Mark Nottingham characterises the old REST vs SOAP debate in a different way. Now it's SOAP vs. HTTP:

you can get any of the listed architectural styles [SOA, RPC, REST] with both SOAP and HTTP.

...[useful table - go see it!]...

When doing so, notice that REST is native in HTTP, and SOAP is native for SOA. Intuitively, I’d rather use HTTP if my chosen architectural style were REST, especially considering how widely deployed and provably interoperable it is. I can choose any number of off-the-shelf, commercial or Open Source intermediaries, caches and libraries to take advantage of REST in HTTP, but the same is not true of REST in SOAP.

My takeaway is: use HTTP if you're doing REST (document based), use SOAP if you want SOA (message based).