I intend to keep this one 2 years. That's how long I kept my Rev B (almost to the day). That said, I purchased the Rev A in February and then sold it to buy the Rev B in November of that year. I took a hit, but the SSD and graphics card in the Rev B was definitely worth it. (For reference, before that I had a Dell Inspiron 600m that I kept for 4 years but upgraded the RAM and CPU along the way before donating it to charity).
The Rev C was barely an upgrade so I skipped it (1.86 to 2.13GHz and little else). The Rev D came with better heat management (less throttling), more RAM, an extra USB port, SD port, better graphics, a bigger and faster SSD, and higher resolution so I figured it was worth it. If Apple keeps the "other" specs the same next year and just swaps out the Core 2 Duo for a Core i3/i5/i7 with Sandy Bridge I'll probably just wait for the second revision with the Core i3/i5/i7.
It definitely gets expensive keeping up with the "latest" but I've always preferred ultra portables (the Inspiron was a brick by comparison).