I came across this today:
Some Indie Facebook Developers Pulling In Over $700,000 A Month
The mass media may be enamored of the rags-to-riches stories of developers on Apple's App Store, but it isn't the only game in town for indie developers to strike it rich. We've gotten word from SocialMedia, a popular ad platform for social network applications, that one of the company's clients pulled in over $700,000 in advertising revenues from their Facebook apps in December alone. Granted, this was spread over 30+ of the client's applications, but the company only consists of a handful of (very prolific) developers.
I found a good article over at softwaredeveloper.com on how to get started... I'm going to read up on the basics listed below:
1. Anatomy of a Facebook Application: A quick overview of a Facebook application; useful to familiarize yourself with the process before starting out.
2. Facebook Developer's Platform: From documentation to resources and tools, this is your one-stop resource for achieving deeper integration between Facebook and your app.
3. Facebook Developer Documentation: A beginner resource for details of the API, as well as the Facebook query and markup languages.
4. Facebook Step-by-Step: Now that you have the background, Facebook's official application guide will walk you through achieving "Hello World" Facebook integration.
5. Facebook FAQ: Answers the questions that Facebook developers most often ask
Now I just need a good idea...I will keep you posted.

I have been trying to get my friends at Centrica to develop a Facebook app that will let users have a high res image on their Facebook pages - what do you think?
Its a good idea...you would need to host the high res images yourself..not on facebook servers
Right - Centrica would host the image - say a free account could have like 5 or 10 MBs of storage, enough for 1 or 2 high res images. And then the widget could be used on Facebook or other social websites (or make it a Flash app that could be embedded anywhere in a web page). The user gets a free hosted high res image for the site, and Centrica gets some viral marketing and branding. The XLImage application has a lot of uses - medical imaging, science, fine art, fashion and retail, etc. - anywhere where people need to see a lot of detail on screen.