Very silly question: have you seen the idea (forgot where, maybe the Nokia idea contest, maybe somewhere here, maybe meego.com?) where the lockscreen photo/backdrop would be replaced according to the weather forecast?

I think that would be great.

Ideas for implementation:
1: user selectable images for each weather type
2: overlay of images (translucent) on top of existing backdrop

For 2, I could imagine that for rain, a bunch of raindrops get put on top of the original image, fog, snow etc, same thing, etc.
For 1, I'd put a snowman picture when it's going to be below 0 Celsius, a meadow with sunflowers when it's going to be sunny and warm, a couch (indoor) when the weather is going to be cold and rainy, a running shower when it's going to be warm (20 C or higher) yet rainy, etcetcetc.