This is a security fix and applies to all architectures. It is serious and should be applied immediately.
Quote from the Errata page:
Incorrect mbuf handling for ICMP6 packets.
Using pf(4) to avoid the problem packets is an effective workaround until the patch can be installed.
Use "block in inet6" in /etc/pf.conf.
If you previously installed rev1 of my binpatch (motd banner looks like "OpenBSD 4.0 (GENERIC) #4: Thu Mar 8 17:30:41 EST 2007"), you should upgrade to my rev2 binpatch (motd banner looks like "OpenBSD 4.0 (GENERIC) #5: Sun Mar 18 01:05:50 EDT 2007").