In short... Yes! GSM modems are generally the recommended device, anyway.
Most should work, but check out the http://www.frontlinesms.com/download/requirements.php page for details of modems we've either tested, or have been told work by users. I personally use the SAMBA, and these are the devices we used during FrontlineSMS development.
Best of luck, and keep us posted on your progress!
might you add some guidelines for assessing a potential modem/phone to the download page? I think I saw in another post that support for the "Hayes AT command" is something to look for...are there other things?
Technically speaking, *all* GSM modems will work with FrontlineSMS. If you are able to try any out before buying one that would be ideal, but support for standard Hayes "AT" commands is the key bit. There is a discussion in the Forum here about using FrontlineSMS in the US: