Until now NSCA has be the only way to integrate passive service checks with Nagios. From a functional perspective it has been working great using the jsendnsca package, http://code.google.com/p/jsendnsca. With the next version we will also support NRDP, Nagios Remote Data Processor. The are some benefits that is nice with NRDP like pure web interface and batch sending of passive checks.
If anyone would like this immediately please send us an email and we could make a 0.4.1 version.