There are three revenue sources for software:
- Sell the software
- Sell consulting/maintenance around the software
- Host the software
In your case you will also have ad revenue from your three web properties that use the codebase.
Open sourcing the software definitely eliminates #1 above and may also eliminate #3 (as Joel mentioned in Podcast #60). I could certainly see FogCreek being able to make a ton of money using the #2 approach - sell the server, installation, configuration, and training for $50,000 plus an annual maintenance fee. Enterprises want the software on their own network - a reason I think the StackExchange idea will not be as successful as it could be.
There will be many advantages of open sourcing the software from a business sense: getting the FogCreek brand out (for Joel), developer good will (you buy products from companies you like), and contributions from open source developers.