I actually emailed Rain Design and asked about this, and they recommended the 15W. Here is what they said in response to my question:
"Thank you for your inquiry. At this point we do not have plans to make an
iLap for the MacBook Pro. However, we recommend the iLap 15" Widescreen
made for the Apple Powerbook for the MacBook Pro. The specs for the iLap
15" W is Width: 13.5" and Depth 9.5", which is the closest to the MacBook
Pro's dimension. Let us know if we can assist you further. Have a great
day!"
(I realize this doesn't really answer your question, but I thought I'd post what Rain Design had to say. In my original email, I had asked if they are going to make an iLap model specifically for the MBP.)