There's a picture of a Chumby controlling an RC car that bunnie put together.  See it at bunnie's RC car.  It gives you a pretty good idea what you can do.

I'm in the process of setting things up to try control an iRobot Create with the Infocast 8".

You probably already know that the Create robot has a DB 25 connector and a DIN connector through which you can communicate with the Create robot's microproccessir with a command language. The DB 25 connector also has a few digital pins, one analogue pin and three PWM pins that can be controlled using the command language through a serial connection. There's also a so-called Command Module containing an Atmega168 which you can buy. You can program it to control the Create robot. The Command Module is unnecessary if  you plan to use a Chumby to control the Create robot.

I considered using just the DB 25 connector to allow the Chumby to control the Create robot and access sensors and servos. I decided it might be easier to use a Phidgets board, the PhidgetInterfaceKit 8/8/8, which will allow a Chumby access to 8 digital input pins, 8 digital output pins and 8 analogue pins through a USB connection. Using the Phidgets board, the Chumby can control sensors, motors, etc. and use the data it obtains to control the Create robot with its command language.
 
Phidgets provides source for drivers for Linux, the Mac, and Windows. The drivers can then be used by a large number of programming languages including C, C++,and Java to control the Phidgets board. I've chosen to use Java since it's support for concurrency is pretty good. I feel that this will make it easier to control a number of sensors and other processes, all running at the same time. If this all works, I plan to try using Scala which runs on the JVM if the Java code works OK. Scala provides even easier control of concurrency with its Actors library.

Fortunately for me, a number of people on the forum have worked out building the Phidgets drivers and installing Java on the Chumby. At this point, the Phidget drivers have been built on my Infocast and a sample C program running on the Infocast recognizes the PhidgetInterfaceKit 8/8/8. Next I'll try using C to actually use the pins on the Phidgets board. Then to use Java to try control the Create robot with the Infocast.

Should you chose to use either Phidgets or Java, the details of adding both have thankfully already been worked out  in several threads here, the wiki and a blog:

building phidgets requires libhid requires libusb on infocast

Using Phidgets on a Chumby/Infocast 8

Oracle's Embedded Java  (me struggling to get Java to work)

Java

Duane points out that the Java mentioned in the Wiki includes a compiler. Embedded Java from Oracle contains only the Java VM and the class libs so you'll have to cross compile if you stick to  that one. Or mix and match.