Yeah - I know people who've used spray or double sided tape even (there's a kind made just for numbers)...

With the new cloth ones we've been using you can throw the whole thing, jersey number and all into the wash if you pin them on. If you keep both numbers on and have enough jerseys to dedicate one to racing then you don't have to keep pinning your numbers.... but you do have to make sure they get dried right away or the pins might rust... (I have a front loader too - I'm not sure I'd do this with a top loader... it might be too rough and rip the pins out)

I know there's been some experimenting with using timing chips at cross races out here. Too bad that doesn't seem to work well with road... (I think too many people finish at once, in too tight packs).