I've written a basic OSD class for the AeroQuad, using the MAX7456 chip to overlay information on a live video feed. Currently, it can display battery voltage, altitude, compass heading, a flight timer and a centre reticle. It uses the corresponding AeroQuad classes for battery, altitude and heading. Here's a short video of it in action:
There was no compass heading display in this video as I was flying with ARG, no magnetometer.
To use it, you'll need a MAX7456 attached to the appropriate pins - marked 'OSD' on the AeroQuad v2 shield. I used the SparkFun breakout board (http://www.sparkfun.com/products/9168). If you use the SparkFun breakout board, you'll probably need to add a 10kOhm pullup resistor between 5V and the RESET pin - mine kept resetting at random without this.
You'll also need to update the character memory for the MAX7456. The MAX7456 has an onboard EEPROM which stores up to 256 12x18 pixel characters. To print the battery, compass, clock icons etc, you need to have these icons in the character memory. I've attached the required character set and some code for loading it in char_utility.zip - this is the tool made by dfraser from the Arduino forums. You'll need to upload the MAX7456_mcm_font.pde sketch to your AeroQuad (ENSURE your motors do not have power!) before sending the ascii_with_aq_font_set.mcm text file via serial. You can use HyperTerm's 'send text file' option under Windows to send the .mcm file, or if using Linux you can simply cat /path/to/.mcm > /dev/ttyUSB0 (or whatever label your Arduino has - could be USB1 etc). It'll take 2-3 minutes to send the entire character set. You'll get progress messages and a message 'Done with file download' sent back via serial when it's finished.
You can get the OSD code from my master branch on Git: https://github.com/EwanColdicott/AeroQuad. I'm new to Git so please yell at me if I've stuffed something up!
Edit 26/5: Mikro has written about performing the character upload process under Windows in more detail here.
The OSD now has a basic attitude indicator, using horizontal lines. There's a video of it on page 3 of this thread.