They've got an IMP at the Computer History Museum on Shoreline in Mountain View that I saw a few months ago... not to mention a whole bunch of other really neat machines. Check out their site at http://www.computerhistory.org/
... mouse over "Exhibits" and click on "Visible Storage", enter the exhibit, click on the 60's-80's link, the "Significant Machines" and then "Interface Message Processor" and you'll be treated with a (small) photo of it and a bunch of info.
As a side note, anyone interested in reading more about the history of the Internet should pick up a copy of "Where Wizards Stay Up Late: The Origins Of The Internet" by Katie Hafner and Matthew Lyon. It was a good read that I recommend (and not just because I learned that three people who played a significant role went to the same high school that I did).