Google Voice basically does the reverse of this; it provides a single "hub" number which you can configure to forward calls to N (I think up to 6) other phone numbers of yours. You can then setup rules for each phone number to not forward to your office phone outside of business hours, or to not forward to your mobile on the weekend, etc.

http://www.google.com/support/voice/bin/answer.py?answer=165...

"And then you could just apply filters to turn off certain phone numbers based on day or hour..."

I think it does do that