>"my workshops which run for 3 hours or more"
At 3+ hours, you're really much better off using a camcorder. (And CNet has a camcorder forum, BTW; see link in left sidebar.)
>"Sony Alpha range but couldn't find how long they could video continuously for."
According to what I've read, all of the ones in your budget (read: everything except the A77) are limited to 29 minute clips in all regions. Also note that the Canon T3i is limited to 12 minute clips in full HD.
The only camera I can think of that does not have a clip length limit in the U.S. and has an external mic input and is within your stated $800 budget is the Panasonic G2. However, I have some reservations as to whether the G2 will last 3+ hours of HD video capture on a single battery/charge. Although, if your workshops are conducted somewhere with AC power, there is an AC adapter available. You're also going to need a high capacity SD card. (I'm guessing at least 32 GB.) And if you're shooting outdoors with the camera in direct sunlight, there may be overheating issues over a 3+ hour run time. (I've never heard of overheating issues with this camera, but I've also never heard of anyone running it for 3+ hours in direct sunlight.)
Mark
My Kodak Zi8 just died so I'm looking for an new Camera. I have 2 main requirements 1as a backup camera to HD video my workshops which run for 3 hours or more. Sound quality is important so I need a jack to use an external mic.
Also
I want to be able to take good still photos. I was looking at the Sony Alpha range but couldn't find how long they could video continuously for. i would like to do both with one camera. ideally for under $800
What can you suggest?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic