Who is the cheapest reseller for pre-assembled Arduino Duemilanove boards, fully compatible cloneduinos included? I plan on teaching a basic class on interfacing with hardware and programming with the Arduino software amongst my friends who like electronics, but want to understand more, especially in regards to microcontrollers, but I can't solder (well) for my life, let alone 10 or more boards.
Chinese vendors on Ebay......
Guaranteed not to be real ones, even when no mention is made of clone or compatible.
Solder 10 boards and you might be a little bit better at soldering. 
Buy the original board or buy seeedstudio boards. At 10 boards you'd probably get discounted. I wouldn't recommend chineese boards. They don't support the forums or development what so ever.
I was looking at the Teensy or Teensy++. It said that it is not fully Arduino compliant. For any Teensy users, what is the disadvantage? I only saw that there was a difference in microcontroller, and otherwise, improvements, such as full speed USB.
The teensy is...well...tiny!
It doesn't have the same footprint nor is it pin compatible with the Arduino (meaning no shields). Also, being a different controller, I have no idea how (or if) you program it with the official Arduino IDE. It certainly can't be as functional as the official Arduino.
Do your friends know how to solder? It could be a fun part of the class:
http://nkcelectronics.com/freeduino-arduino-diecimila-compatible-board-complete-kit.html
That comes out to ~$19 each (at the discounted rate for 10), and it is 100% compatible with the original Arduino.
The teensy is a great arduino compatible board, but I wouldn't recommend it for beginners, because some of the arduino libraries won't work without modification. And of course it is not shield compatible.
It has some advantages over the standard arduino. It can function as a USB HID device- it can act as a USB mouse or keyboard without additional hardware, and without needing any drivers installed on the PC. And the Teensy++ has memory and I/O similar to the arduino mega at less than half the price.
-transfinite
Speaking of seeedstudio....
Are they legit? Has anyone ordered from them?
What is the shipping like? Weeks and weeks to get to the states?
Some of their prices are just ridiculously cheap, but I wonder if it is too good to be true and there is going to be a lot of intercontinental returning going on.
I've bought from Seeed studios, shipping is predictably slow, but they do ship. The stuff I bought worked without issue.