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, Iâ€™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).