Mining as a screensaver
In the interests of keeping people's power bills low, I'd recommend not bothering to write anything that uses CPU mining as the bitcoins earned will not even come close to recovering the cost of the electricity.

If you can get it working with ATI graphics cards though, it would be quite a neat idea. It would also be a good way for charitable organisations to earn donations without people needing to put in their credit card details. Eg a donate-to-wikileaks screensaver.

However, there are also ethical issues to consider - not only does mining have a substantial electricity cost (where I live, it can be as high as $45 a month with my 5850 card), it also has a substantial effect of shortening the life of the hardware doing the processing. The average Joe Bloggs should really be made aware of those issues before they install &amp; run mining software.