Linda, who owns Deseret Designs, has developed a DVD showing you exactly how to do this! You can find more info by clicking HERE!
Even more info from the cakecentral.com site:
I use a Cricut to cut gumpaste. First of all, you have to remove all of the tackiness from the cutting mat, otherwise you can't get the paste off after it's cut. (I literally rub it all off with my fingers then wash the mat in hot water.) To hold the paste in place, I apply a thin smear of crisco to the mat. I roll the paste thin, to a #7 on my pasta machine, and let it dry slightly. It works best with old gumpaste actually. I trim the paste so that it does not extend beyond the area that paper would normally be on the mat. I set the blade depth to a 4.
I get very clean, precise cuts except when the blade needs to be cleaned (after about 24 uses). I just remove the blade holder and use a straight pin to flick out the build up of paste. It comes right out, and I'm up and running again.
You can read over 17 pages of a message board thread on this topic over at cake central by clicking HERE!
Is there anything the Cricut can't do??