Check out eBay's heap tables dynamic rows patch

See http://code.google.com/p/mysql-heap-dynamic-rows/

Patch is on MySQL 5.0 codebase.

Notes on the patch (info from Harrison Fisk):
 - Applies to VARCHAR cols, text/blob is still not handled.
 - it is only enabled if you use a special create table option
 - it only works for variable width columns in a table after any indexed columns
 - thus not supported for automatic temporary tables - investigate this, see if col swapping might be feasible!