Heh, don't I know it. My P4M-based laptop guzzle its secondary battery in 2 hours flat -- and it's primary battery is totally knackered (lasts half an hour on a good day).
AMD64 isn't exactly brilliant in that regard either. Some of the regulars in #mipslinux (irc.freenode.net) often poke fun at the sort of power requirements imposed by these CPUs.
Unfortunately, PowerPC-based laptops are rather exxy (not to mention proprietry). One company did produce a "SparcBook", that is, a laptop based on Sun's SPARC architecture... but they're even worse (try > $8000 for a machine).
And the closest I've come to MIPS-based laptops is the early Windows CE-powered units, many of them running NEC VR41xx-series CPUs (64-bit MIPS3), Phillips Poseidon (32-bit MIPS1) or the Toshiba TX39H (also 32-bit MIPS1).
So yeah... take your pick. We've really got a lot of choice.