I had seen this mentioned, and there is even a documented procedure that is related (Integrate Designer 3.0 Into Existing Eclipse Installation) and another article mentioning it feasibility/inabilities (Importing a Driver Export File Without Password Prompts). When I got my new iMac (27" core i7) last month, I wasn't satisfied with the performance of my current solution ('ssh -X'ing Designer from a lab machine). So I tried installing Designer on my Mac a few times. There are only a few noted steps, and never got it to work (btw... I think the steps are outdated and incomplete). I am sure any one that is a little familiar with Eclipse is laughing at me right now.
Figure 1 - Eclipse with Designer for Identity Manager perspective
But for anyone, like me, whose only previous interaction with Eclipse is through a pre-packaged product like Designer (or Analyzer), these steps are very easy.
Figure 3 - Show Eclipse package contents
Figure 4 - Modify eclipse.ini file
Figure 5 - Select Designer perspective