You could have a look at Loxone - they have integration into KNX and they also support RESTful automation.
Besides that you could probably have a look at devices like recently announced Gira X1 or any KNX/IP gateway - many vendors provide some kind of interfaces. For instance, have a look at VisuControl from MDT.
I haven't found any complete products. However, you can download a complete software image for the Raspberry Pi of the KNX Web Service at KNX.org. It's free. The registration can be done with bogus information and 10 minute mail. Figuring out that the password has to be numbers only wasted some time though.
Anyways, after downloading that info package the ...