Good thread, iv tried to use fl studio rewire a few times never really figured it out. 

Thanks all, I'm still struggling.  Bigdogs - I can't bounce to tracks because there is nothing in the track view of the FLStudio track.  Here's what I'm doing.  Inserting FLStudio as a VSTi in a "Simple Instrument Track".  I go into FLStudio and create a simple beat.  When I go back to Sonar and click "Play" I can hear the FLStudio beat being played.  However, I can't record what I'm hearing.  If I click Record in Sonar, I can again hear the FLStudio beat, but nothing gets recorded?
 
I've tried this in Reaper and when I click record it records the FLStudio beat as audio into the track in Reaper.
 
Anyone else?

Even thought the track is empty, you have to select the region of the track to bounce, otherwise Sonar doesn't know what to bounce. If this doesn't work let me know and I'll take some screenshots showing the process and settings

Do you want to record the FL Studio audio? Or the midi??

How about not even bothering using FL Studio if you're going to use it how I'm assuming you're using it? Are you using it to set up quick drum patterns on the Step Sequencer? Here's a quick method I created that should allow you to skip using FL for that purpose:
 
http://screencast.com/t/chXxG2gyivin
 
I used Session Drummer to load my sounds from the Browser. From there I opened up the Step Sequencer and deleted all of the unused rows just to keep things organized. I laid down a simple drum beat, that's it.
 
If that's not similar to what you were intending to do with FL Studio just explain to me what you're trying to do exactly and I'll figure it out for you.

Sonar does not record VSTi without third party help it bounces the audio. The same results most of the time just different method. Just in case you did not understand the directions given: select the instrument track and then left mouse on the timeline and drag your mouse across the timeline this selects the range that will be bounced when there is no midi in sonar. You can figure this out by just playing your song and look to where it ends in the timeline. The timeline will be highlighted choose bounce to track from the 'track' drop down menu located just above the track pane and to the left of the timeline and audio will be bounced to what ever track you select. I haven't used FL Studio in a while, back around sonar 3, 4 or 5 rewire worked fine for me.

Thanks to everyone for their help.  Bigdogs / rabeach - I got it working, many thanks.