I am a teacher at a school which is putting on a workshop for early teens (late secondary to early high school). We would like to teach them basic programing, debugging and problem solving. The software should be compatible with Windows XP, VISTA and MAC OS X and programmed via the USB. We are looking at robots in the $300 dollar range in order to have them be affordable to the widest possible range of children. What do you suggest?
LEGO Mindstorms is definitly a good choice but it remains a ‘toy’. An Advanced toy but a toy… For early teens, it’s a the best choice.
But, take a look to VEX products :
robotshop.ca/vex-protobot-autonomous-kit-11.html
And also, my personnal best choice : iRobot create
store.irobot.com/family/index.jsp?categoryId=2591511&ab=CMS_IRBT_CreateSuperCat_LearnMore_110408
What is iRobot Create? 
iRobot Create is an affordable programmable, mobile-platform robot. It is pre-assembled so students, developers and aspiring roboticists can develop new robot applications without having to first build a mobile robot from scratch. Create is used by developers to facilitate the creation of new robots and educators to teach students about programming and robotics.
Also, I think it’s a solution for the futur. And it’s a floor cleaner already used by a lot of people.
For early age teens, go with lego mindstorms.
Jeff
I have also had success with students using:
BOE-Bot (Board of Education)
$170 USD reobot
$30 USD Floor sensors
Programmed in a simple form of Basic called PBasic
and
Pololu 3PI from RobotShop.com
Fast line follower and maze solver
$117 USD
Programmed in C
See this link for Youtube videos of my student’s projects:
richardvannoy.info
In terms of education, the LEGO Mindstorms NXT really sets the bar high.
These are widely used and recognized by teachers as a godsend. Students are engaged when using the NXT kits as they allow maximum creativity.
Carnegie Mellon University has also developed a LEGO curriculum that may interest you.
The newest NXT software v1.1 is compatible with: Windows XP Professional or Home Edition with Service Pack 2, Windows Vista (32 and 64 bit) and Intel-Based Macintosh systems. Some people use it on a Linux host via VMWare Server running the Windows environment (XP SP2). Some people have used the NXT “brick” with Windows mobile but this is highly experimental.
You may also be interested in re-using your older kits with a USB to Serial converter.