With the update of Logic 5.2

Monitoring with effect plug-ins
Record-enabled (armed) audio tracks with assigned audio inputs can now be monitored with effects plug-ins inserted into the armed audio channel. In this case the effect plug-ins are monitored but not recorded. This can be useful during an overdub session for example. If you like to record the audio track with effects, insert the effect plug-ins into the corresponding input channel instead (see below).

Example: During vocal recording many artists prefer to monitor their performance with a little bit of reverb while the track is actually recorded dry. To do so insert a reverb plug-in into the audio channel, arm the track and start recording. The reverb will be part of the monitor mix but will not be recorded.

It is possible to combine pre-processing and monitoring with effects. You can for example record a signal with compression destructively by inserting a compressor into the respective input channel while monitoring the same compressed input signal with reverb that is not recorded.

For this to work, the ”Software Monitoring” preference for the particular hardware must be enabled in its driver panel. (Menu: Audio > Audio Hardware & Drivers.)