You could, but you would letting out your SIP information to the public internet. You would need to open a pin hole for 5060 or 5061 and allow that traffic to go through your firewall. CUPS is not a fan of firewall SIP inspection also. You may find presence bubbles on/off or go gray. I dont have a timeline, but we know that the Jabber cleint will have the lightweight anyclient built into which will automatically build a vpn tunnel in the background back to Corp. This will then turn your IM client into a seemless client. Right now, the end user would have to launch VPN, then IM/softphone would work. With the next version, it will automatically work for them. Similar to what you see on the iphone/ipad and soon android.
As for now... stick with SSL vpn client or something back to corp. You may also try some sort of XMPP gateway in your DMZ. I have not tried it, but may work.