A little while ago I posted about my problem trying to get bcm5754 
working on an OptiPlex 745:

http://mail-index.netbsd.org/current-users/2007/01/09/0003.html and 
http://mail-index.netbsd.org/current-users/2007/01/09/0005.html

I since had a brief dialog with Jonathan Stone who suggested some 
changes to the broadcom support but they made no difference so the 
current behaviour is still that packets get out but it doesn't appear 
to notice packets coming back.

So I bought a cheap realtek pci card and plugged it in and it appears 
to do the same thing - I can see arp packets coming out of it but the 
machine never notices the response packets sent back.  However if I 
move the realtek card to an Optiplex 620 it works fine.

Anyone got any idea whats going on?

cheers
mark