Yes, there are sirfergy. You would have to use an analog cell phone though (since a modem translates analog to digital and vice versa). This would be extremely slow (9600 kb/s or slower), and I would not suggest doing it this way. Most of the current digital cell phones (with SprintPCS, AT&T, etc.) have a modem type of device built into the cellular network. You just need to purchase a serial cable (for $60 or so!), and your computer will think it's a regular external modem. Currently, the maximum speed you can connect at is 14.4 kb/s. Your computer will report a connection of 19.2 kb/s because that is how fast the connection is between the computer and the phone. I've done this with SprintPCS and it works great. They even have free software available to compress the data so that it is comparable to a 56k modem connection. In the not too distant future, high speed access (similar to DSL/cable) will be available, and will work with all current phones, according to SprintPCS.
I hope this helps!