POP3 and IMAP4 are installed by default with Exchange 2003 but are not enabled. they can coexist in tandem with each other without an issue. The protocols can be enabled using the ESM; expand servers, server name, protocols, expand the desired protocol & right click it then simply select start to fire up the virtual server. Configuring the protocols is just the same as configuring the SMTP protocol. Security wise you would be opening more vectors of attack to your exchange server as your opening ports on your firewall, consider using front-end servers tying down the security on the exchange servers, use the security & config wizard (part of 2003 server) to reduce the running services, open ports etc on the exchange boxes, general security measures that you should be taking with any server that will be accepting coms from the t’interweb.
My question to you is why do you need to have POP3 and or IMAP4 running?