So, just as a reference here are the keys we're currently seeing as ones that will not be converted to gold or removed upon release of the patch -- if you have them they'd be put back into your normal inventory. This isn't necessarily a final list, and it's possible we could cut the list down even further before 4.2 releases.
Grim Guzzler Key (BRD)
Secret Safe Key (BRD)
Dark Keeper Key (BRD)
Relic Coffer Key (BRD)
Jump-a-Tron 4000 Key (Nagrand)
Kolkar Booty Key (Barrens quest)
Ethereum Prison Key (Netherstorm – Blade’s Edge)
Captain's Key (Tanaris)
Any Blacksmith keys (e.g. Titanium Skeleton Key)