Some time ago Josh Berkus wrote about possible changes in VACUUM FULL.
Now these changes came to life. By now, I mean 6th of January, when Takahiro Itagaki committed his patch:
Log Message: ----------- Support rewritten-based full vacuum as VACUUM FULL. Traditional VACUUM FULL was renamed to VACUUM FULL INPLACE. Also added a new option -i, --inplace for vacuumdb to perform FULL INPLACE vacuuming. Since the new VACUUM FULL uses CLUSTER infrastructure, we cannot use it for system tables. VACUUM FULL for system tables always fall back into VACUUM FULL INPLACE silently. Itagaki Takahiro, reviewed by Jeff Davis and Simon Riggs.