There are several apps that will allow screen captures to be accomplished over the USB, but I don't know of any that will do it from a stand alone Blackberry.
I use JL_Cmder which can be downloaded from our downloads section as it has other useful functions. There are others, but I haven't tried them. Check the third party software forum for information on them.
Hope this helps.
~via My BB Curve (wap.pinstack.com)~