Discussed in
this thread. As well, there is nothing in the rules against magnets, but there are a few things to take into account.
1. You will be hard pressed to find manageable rare Earth magnets that will hold your entire robot.
2. Electromagnets, while possible to design, may not fall under custom circuits. As well, keep in mind that power is cut at the end of the match, in which you risk up to an 8 foot fall.