Since a few weeks i use zeroshell as a UMTS/WLAN Router. I use a linksys UMTS Router connected to eth0 and a D-Link AP connected to eth1.
It works fine.
Yes, I know that in your case works fine, but you need of two box: the UMTS router and the Zeroshell router. If Zeroshell will directly support USB 3G modem you can eliminate the additional UMTS router.
In near future Zeroshell will also have Asterisk with which it will able to act as a VoIP PBX. In this way will be possible to attach traditional analog PSTN phone to a Zeroshell box with a FXS card and use the mobile phone operators.