I'm running LMDE64, but using sid repositories (I'm impatient, testing takes too long).
My work-around was to download gtk2-engines-murrine_0.98.1.1-2_amd64.deb from the packages.debian.org website and install it manually. Once I installed the gtk2-engines package, murrine-themes updated normally through apt/synaptic. I'm guessing this shouldn't break any functionality for future mint-artwork-debian updates (not sure how version specific the dependency is).
Not that it matters much as I'm not currently using a murrine theme, but nice to know I have the option to down the road.
↯Acer Aspire 7730 17" laptop ↝ Intel Core Duo 2.00Ghz x2 4gig DDR2 RAM ⤗ Intel Mobile 4 Graphics ➫ Kernel: 3.2.0-14-liquorix ZEN SMP PREEMPT x86_64 Linux Mint Debian Edition using Sid Repositories ⇨ KDE 4.7.4 ⟿ Installed: 11/23/2010↯