I'll post my solution here in case anybody else runs into the same problem.
I called IBM who told me:
1. Send the unit back and they will remove the stuck pen in 3 - 5 days under warranty.
2. Do not under any circumstances try to disassemble the ThinkPad to do this yourself.
Of course, I chose #2
If you need to dislodge a stuck pen, power down and remove the battery. Turn the unit over and remove the four screws on the back at the bottom. Remove the next row of four screws above the bottom row.
At this point you should be able to
very carefully start to separate the top and bottom half of the lower case. Start at the center near the latch and work it open very slightly. You should be able to see the end of the pen. Push it out; it will offer some resistance but then the latch will release and it will pop out. Next put all of the screws back and remove the hard disk drive cover now that the pen is not blocking removal. The pen docking mechanism is in the hard disk cover.
I found by examination that the defective pens have a very slightly wider clip (the part that fits over a shirt pocket). You can compare the width to that of a good pen to see the difference. Get out your X-acto knife and shave a fraction of a mm from each side of the clip. Start above the two protrusions at the bottom end of the clip and go back up toward the top of the pen about 10 - 15 mm. Test by docking the pen in the mechanism. If it gets stuck again you haven't shaved enough off. Continue until it pops in and out of the dock with no resistance. Clean it up with very fine grit sandpaper and you just saved yourself $85 for a new pen.
Even though Lenovo released a few pens with this defect, their warranty only covers the pen for a year. My mistake was putting it away for two years before trying it out.