Mmm, it would be kind of cool if you could automatically set it up somehow so that, if and only if you were to be logged into Skype (and possibly also only if you didn't have cell reception), calls would automatically forward to Skype. So that, say, when I'm in some offices in the hospital writing, where the cells don't work, Skype would be an auto backup...
Of course, I don't even *have* SkypeIn.

And I can't think of a good way to create that level of sophistication.
You can do "if unavailable" call forwarding with cells, but there's no way for the cell to know if Skype is available...
I guess, you could do this:
1) Set cell service "if unavailable" to your SkypeIn
2) Set Skype's "if unavailable" (I think it has one?) to your cell's voicemail number... although, I'm not sure if they would end up at your voicemail box if you did this, and not just at the general prompt for your cell service's voicemail system.