Saw this on Droid Life and it may be found in a bunch of other threads, but here is the fix:
Instructions...
*Note 1* - If you don't have Terminal Emulator, download it from the market.
1. Open Terminal Emulator and type in the follow commands:
For JRummy ROMs:
su
mountsys
rm system/app/Maps.apk
cd system/bin
sh pm uninstall com.google.android.apps.maps
unmountsys
For all other ROMs:
su
mount -o remount,rw -t yaffs2 dev/block/mtdblock4 system
busybox rm system/app/Maps.apk
cd system/bin
sh pm uninstall com.google.android.apps.maps
cd ..
cd ..
mount -o ro,remount -t yaffs2 dev/block/mtdblock4 system
sync
2. You can now attempt to install the Google Maps update.
3. If it fails the first time, just cancel the download and try again. If it fails again, reboot and try.
4. Done!