Using Rewire with Logic 8 is even easier than it was with Logic 7.
1-Create a new "external MIDI" track by clicking the "+" in the arrange window.
2-Click on "Media" in Logic's toolbar, select the Library Tab and double click on the "Reason" folder. Reason will launch.
3-Create some instruments in Reason... for exemple a mixer and a subtractor.
4-Back to Logic. In the arrange window, select the "external MIDI" track you created before and choose the SubTracktor in the Library Tab.
5-Go to the mixer window. Select "Options - Create New Auxiliary channel strips... You can choose Mono/Stereo and the Reason input. Select "RW:Mix L/R".
6-Make sure the aux level is up... and have fun!
Also, if you need to send specific inst to different aux, just select another Rewire channel in Logic's Aux input. Each channel correspond to a Reason's output. If you send a mono inst to a Reason output, make sure the Logic Aux is set to mono.
Just tell me if you need more info!