Would it be okay to plug a 90W Thinkpad power brick to a X200? It originally came with a 65W adapter but it has stopped working. So I also have a 90W adapter that came with a T61 - can I use it without any problems?
Check the voltage rating on the power supply and on the laptop. If it is the same, and socket on the notebook matches the plug on power supply, it is okay: while it can provide more power than needed (90W in this case), it will behavior just as the 65W one when plugged in to that laptop.