It'll either be titanium or stainless steel. Depends where it was done. Friend has a titanium plate in his ankle (op done in France); I have a stainless one in my wrist with the op done in the UK.
It's apparently MRI safe, and sadly doesn't set off the metal detectors at security checkpoints. I can also feel the plate, although since the screws go right the way through the bone you can also see the points protuding against the skin on the top of my wrist...
If anyone stumbles across this post later, it presented no problem for my Class 1.