Not seeing it here, after updating from (about) 20-May alpha to 20-June alpha. x86-64, physical machines, no packages. Also cannot reproduce it with i386 or x86-64 virtual box machines.
When it hangs, is there anything interesting in the logs or on the console? What about the last few lines of /cf/conf/upgrade_log.txt? Mine typically look like so:
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local/livefs/lib:
modification time (Tue May 20 15:58:13 2014, Fri Jun 20 09:10:24 2014)
Checking for /tmp//tmp/post_upgrade_command.php...
Running /tmp/post_upgrade_command.php
Adding serial port settings ()...
Reading /boot/loader.conf...
Fri Jun 20 12:23:20 EDT 2014
Firmware upgrade is complete. Rebooting in 10 seconds.