Invalid magic number in log file?

When starting up a postmaster from an older build on a database initialized
with a newer build I'm getting the following errors. I think I saw the same
thing earlier going in the opposite direction too. Shouldn't there be some
earlier errors firing from the control file version number?

stark(at)oxford:~$ /usr/local/pgsql/bin/postgres -D /var/tmp/db
LOG: database system was shut down at 2007-05-14 19:47:51 BST
LOG: invalid magic number D061 in log file 0, segment 0, offset 0
LOG: invalid primary checkpoint record
LOG: invalid magic number D061 in log file 0, segment 0, offset 0
LOG: invalid secondary checkpoint record
PANIC: could not locate a valid checkpoint record
LOG: startup process (PID 9590) was terminated by signal 6: Aborted
LOG: aborting startup due to startup process failure

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com