SIP SIMPLE extensions get slowly into IM & Presence market, however, they are more visible lately than few years ago. One of the key components in XCAP server - an XML storage engine that uses HTTP for transport.
Kamailio 3.1.0 is first open source SIP server that includes an embedded XCAP server - functionality provided by xcap_server module. That means a very tight integration with the SIMPLE Presence server offered by Kamailio for quite some time.
Although XCAP specs specify HTTP as transport protocol, the implementation in Kamailio can work fine with XCAP documents transferred with SIP (over TCP, TLS and even SCTP or UDP).
All these together make the implementation of a SIP SIMPLE server simpler than anytime before. You get a compact solution, within a single application. However, worth to mention that Kamailio can be used as a stand-alone XCAP server, with other SIP application servers.
There is a complete tutorial about how to deploy Kamailio with embedded XCAP server, using SIP Communicator as softphone. You will discover that the additions to default Kamailio config file are pretty minimal, straightforward and independent of SIP routing logic - see the tutorial at:
xcap_server module documentation: