Two things:

* You're going to have to support your SOAP endpoint anyway, so only maintain libraries for your SOAP endpoint (at least for now). Having a PHP library for SOAP and a PHP library for REST will just confuse people to no end.

* Put up a form on your web site that says "Put your email in to get early access to our REST API" and then see if anyone actually bites.

Just because someone on a survey said they wanted REST instead of SOAP doesn't mean that you have to run off and do it, or that they wouldn't necessarily use the SOAP API if they actually had to solve a problem with your API.

We built some really interesting stuff on SOAP and WS-* in the 2002-2003 era, and it's always fun to see development trends go in circles.