In the process of recording a vocal with numerous takes, I now have about 25 "take lanes".
Most of the clips don't have any timing overlap, so is there a quick way to compact these down into a fewer number of lanes?

If you're left with only clips you want to keep and the rest are muted, select all the clips and bounce them.

Anderton
If you're left with only clips you want to keep and the rest are muted, select all the clips and bounce them.

Thanks, but presumably that will leave me with one large clip and I wanted (for the moment at least) to retain the individual clips 

I assume when you say 'most clips' you mean you have a mixture of clips that overlap and most that don't. You could drag and drop those that don't overlap, and you want to keep, into one or three take lanes then delete the empty lanes that you then have left. I'm not sure there is a quicker method than this when you have such a mixture, but it should only take you a minute at most.
 
Otherwise, Craig's suggestion is a quick way of bouncing selected clips to new lanes. You still have the opportunity to drag over and select parts of that new clip, and Sonar will automatically slice it up at those selections.