Separate firewall/VPN, or integrated? References appreciated

[Bob George <mailings01 () ttlexceeded com>]

Are there any authoritative references on the subject of combining 
firewall and VPN server functionality (i.e. Nokia/CheckPoint) versus 
separate devices (i.e. Cisco PIX, VPN3000)? There's plenty of 
vendor-speak, and I've got my own ideas about KISS and firewalls, but 
it's nice to have a respected reference to cite as well.

Thanks,

- Bob