Two networks at the same time

Hi,

What are the likely problems and security risks of the following?

PC connected to the LAN using a wired ethernet connection. Server 2003 AD
domain and ISA prevents using pop3, smtp, etc.
A wireless network connection on the same machine connects to the Internet
via another broadband line with no ISA or any similar restrictions, just the
basic firewall.
The user runs the usual apps on the LAN and at the same time connects
remotely to other networks using the wireless connection, and operates an
smtp server using the same.

What is the risk to the LAN?

Thanks in advance
SW