>3:
 >an0 at pcmcia1 function 0: Cisco Systems, 350 Series Wireless LAN Adapter
 >an0: Cisco Systems 350 Series (firmware 4.25.30)
 >This one gives me loads of these under heavier traffic than just ping or
 >interactive ssh.
 >an0: discarding oversize frame (len=1516)
 >an0: discarding oversize frame (len=1516)
 >an0: discarding oversize frame (len=1516)

I had the -exact- same issue with that card under a slightly older version 
of -current (1.6ZK).  The only thing I could find to fix it was to lower 
the MTU on my wireless router.  For some reason, (I assume it's part of the 
802.11 standard, though I haven't checked) no matter what the MTU is set 
to, it always tacks on 16 bytes (I think the len is in bytes?).  Dropping 
it to 1484 should fix that problem, though I set mine to 1200, just to be 
on the safe side.  However, this does bring up a question.  Could the card 
have its MTU raised above 1500?  ifconfig wouldn't let me do it, so I don't 
know if that's a hardware, software, or standards based limitation.