I always heard that putting a bit of DE in a cartridge filter would shorten the life of the cartridge but I just found this tidbit on the Pentair websire in their FAQ for pool owners:

http://www.pentairpool.com/faq-faq_filters3.htm

"How does the micron filtration rating for a D.E. filter compare with that of a cartridge filter?
Cartridge filters are rated at approximately 6-10 microns. D.E. filters are rated at approximately 2-4 microns. So you will get better filtration with the D.E. filter. You can make a cartridge filter a little more efficient by adding 1/4 to 1/2 pound of D.E. through the skimmer. "

Anyone have any ideas on this or have tried this?