The thing that attracts me to Arduino is the number of addons such as gps, camera, bluetooth, usb, servos, etc available. Do any of these ARM9, etc have a similar community?
If I can't find another platform, can I connect and control multiple Arduino's from a single computer like an ultraportable?
You can do whatever you want, but you'll need to come up with the implementation. I don't think you'll get much by using multiple Arduino's in parallel, though.
At the very least, you get real multithreading which can make things easier for a beginner, let's say one Arduino to read analog sensors, and a second to process and display the information.
unless there are libraries out there for linking two or more Arduinos via serial/I2C/SPI?