This seems to be the week to revisit old posts. Back in November, I told you about how you could clip various types of referenced files, including DWF, and DGN, and images.
It's time to come back to that, because in LT 2010, that ability has been expanded to include DWG and PDF files.
The Clip function, on the Reference panel of the Insert tab, enables you to clip any referenced file -- DWG, DWF, PDF, DGN, or image -- to a boundary of any shape. You can even invert the clip, to hide the part of the reference that's inside the clipping boundary intstead of the part that's outside.
For the command line fans, type CLIP to launch this, not XCLIP -- XCLIP will work, but only on DWG files. CLIP works on any kind of reference.