I need a heated surface for a project and was thinking of using one of these PCB heater beds. But I'd need to drill a few holes in it - can I do this without stopping it working? Thanks
If it is constructed like this one then at best you'll have dead stripes where you've drilled through conductors; at worst you'll short some adjacent stripes together and you'll (probably) get hot spots.
PCB layout software is cheap; you may want to see if you can get the part made as specified in the Wiki I link to (or use a service that uses gold flash rather than tin plate -- gold flash is much thinner, so it won't disturb the resistivity of the copper as much).
If you can, design a board with holes. Run an annular ring of copper around each hole, with stripes going into and out of the rings. Current will conduct around the holes, then go back into stripes where it'll do it's job of heating up the board. Without a lot of painful calculation the heating won't be perfectly uniform, but it should be pretty good -- or you can do a lot of painful calculation, and get it spot on.