I'm experimenting with making magnets of some of the elements of the Wireframe Stencil to use for prototyping. Was an interesting experiment printing and cutting these. Next time I think I'll have to laminate the sheet first, and then cut. The lamination might make it possible to write on them with dry erase marker. If you write directly on the magnet sheets the ink will stay permanently I'd think. The surface seems kind of porous and a little textured.
When I get a version that seems to work, I'll post PDFs to the Tools section that you can download for free, or as donationware. You can then print on Avery 3270 Magnet Sheets.
http://www.flickr.com/photos/jibbajabba/3658427576/?addedcomment=1#comment72157620359280161