<p>I have a large quantity of clearcase data which needs to be migrated into perforce.  The revisions span the better part of a decade and I need to preserve as much branch and tag information as possible.  Additionally we make extensive use of symbolic links, supported in clearcase but not in perforce.  What advice or tools can you suggest which might make this easier?  </p>

The first step is to decide if you need to migrate everything, or just certain key versions.  If you only migrate the important versions (releases and major milestones) you'll end up with a much simpler history in Perforce, without losing anything important.  Then ClearCase can be keep as a historical archive in case it is ever needed.  (Unless IBM has changed things ClearCase licenses do not expire when maintainance runs out, you just lose the right to new upgrades and patches and acces to support)
Keep in mind that Perforce does not version control directories and does not keep a full per-element version tree - this means a 1:1 with exact results is going to be impossible.  Recreating the important snapshots is a much more achievable goal; keeping everything may be impossible, as Perforce lacks features ClearCase relies upon.
To see what Perforce says about the miration, check out
http://perforce.com/perforce/ccaseconv.html
This explains the key differences and covers a few approaches you can take.