Just another opinion--
We did Land/Sea in October, 4 days WDW first, then cruise. Let me start by saying we had a great time!!! We booked it that way for the reasons everyone has already said--get exhausted at WDW and then relax on the cruise.
Well, now that I've done it, I have to say that we felt like sleeping through our whole cruise!!! We were so tired every night we didn't even do the buffets at night and just went back to the room after dinner.

(BTW, we are night people!) It didn't ruin our trip, but I think we would've gotten so much more out of it if we had cruised first, and then had the energy to do the parks.
Think of it like we do the beach: when you get there, all you want to do it sit on the beach all day!!!

After a couple of days, you're more interested in seeing the area and snooping around (at least that's the way we are!)
I think it will help with packing too, since the more important packing issues are with the cruise; it was hard to keep all that out of the way and make sure we had everything together after the parks in order to get on the boat. But if you got all your dress stuff out of the way, you just have your t-shirts and shorts to mess with at WDW!
JMO
Amy