Since change 521d6b14, SRC_URI checksums are not being checked at all.  I believe that this is because the .done stamps are being created manually instead of calling the update_stamp() function which is the place that checksums are verified.

It's easy to reproduce this - just mangle the checksums in a recipe and run through patch, e.g. I mangled the recipe for busybox, then ran
  % bitbake busybox -c cleansstate
  % bitbake busybox -c patch
which yielded this output:
OE Build Configuration:
BB_VERSION        = "1.15.1"
TARGET_ARCH       = "arm"
TARGET_OS         = "linux-gnueabi"
MACHINE           = "beagleboard"
DISTRO            = "poky"
DISTRO_VERSION    = "1.1+snapshot-20120417"
TUNE_FEATURES     = "armv7a vfp neon cortexa8"
TARGET_FPU        = "vfp-neon"
meta              
meta-yocto        = "(nobranch):4d9f4d6ac25f39fe6d5491d05c7a26195bab648b"

NOTE: Resolving any missing task queue dependencies
NOTE: Preparing runqueue
NOTE: Executing SetScene Tasks
NOTE: Executing RunQueue Tasks
NOTE: Running task 8 of 10 (ID: 2, /home/local/poky-master/meta/recipes-core/busybox/busybox_1.19.4.bb, do_fetch)
NOTE: package busybox-1.19.4-r1: task do_fetch: Started
WARNING: Failed to fetch URL http://www.busybox.net/downloads/busybox-1.19.4.tar.bz2;name=tarball
NOTE: package busybox-1.19.4-r1: task do_fetch: Succeeded
NOTE: Running task 9 of 10 (ID: 0, /home/local/poky-master/meta/recipes-core/busybox/busybox_1.19.4.bb, do_unpack)
NOTE: package busybox-1.19.4-r1: task do_unpack: Started
NOTE: package busybox-1.19.4-r1: task do_unpack: Succeeded
NOTE: Running task 10 of 10 (ID: 1, /home/local/poky-master/meta/recipes-core/busybox/busybox_1.19.4.bb, do_patch)
NOTE: package busybox-1.19.4-r1: task do_patch: Started
NOTE: package busybox-1.19.4-r1: task do_patch: Succeeded
NOTE: Tasks Summary: Attempted 10 tasks of which 7 didn't need to be rerun and all succeeded.

This should fail (and did before the changeset mentioned above).  Tested in
master (04d6aa1) and 1.2_M4.rc4 (4d9f4d6)