We branched RELENG_2_0 for the upcoming 2.0 release so that some developers could continue post-2.0 work on HEAD without committing code that would hinder the 2.0 release. Because of this, the URL for snapshots has changed. The previous URL had pfSense_HEAD, and it is now pfSense_RELENG_2_0.
visit System > Firmware, on the Settings tab, and pick the snapshots URL appropriate for your hardware (i386 or amd64) from the "Default Auto Update URLs" drop-down menu in order to pick up any new updates. This drop-down is pulled from our update server, and I fixed the URLs in there, so choosing the right selection will re-copy the proper URL into the settings box.
If you wish to put the URL in manually, it would be one of the following (depending on whether you are i386 or amd64):
pfSense 2.0 RC i386 Snapshots: http://snapshots.pfsense.org/FreeBSD_RELENG_8_1/i386/pfSense_RELENG_2_0/.updaters/
pfSense 2.0 RC amd64 Snapshots: http://snapshots.pfsense.org/FreeBSD_RELENG_8_1/amd64/pfSense_RELENG_2_0/.updaters/
I have changed the default URL for new installs, but a snapshot does not yet exist with the updated URL built-in. If you select the new URL or enter it manually, that setting will carry over. Once the next new snapshot is uploaded, new installs should look in the right location, and those without an update URL chosen at all should be OK, but unless you update manually, you still need to fix the update URL as described above.
I corrected the URLs on http://snapshots.pfsense.org/
to point to the new location as well, you can download manual firmware update files directly from there if you like.