Ever since my desk phone went VoIP, I have been saying how I love forwarding my desk phone to my Treo and only giving out my desk phone number. So, basically, whenever anyone calls my office phone, it forwards through to my Treo. I can turn this off anytime -- although, I generally never do. This is great for multiple reasons: I only ever have to give out one number, I don't have to tell everyone my cell number, I can get calls from anywhere, if my cell number changes, it's seamless, etc. But, it looks like there is something even better coming out.
Grand Central is a service that has just been acquired by (you guessed it) Google. I haven't used it firsthand yet, but here is my superficial understanding of it. Once you set up an account, you can enter all of your phone numbers - in my case, a couple of cells, office, etc. In return, you get one number. You would distribute this number to all of your contacts.
Here is where the cooler part starts. On the web site, you can set up advanced calling rules. Examples: if Mom calls, route her call to my personal cell. If John from work is calling, send it to my Treo. If I'm waiting for a call from John, ring all of my phones. It also has the ability to set SPAM rules for telemarketers. You can swap phones in the middle of a conversation. Pretty awesome.
Caveat emptor: As much as we love our Google apps, we have to remember they are in this for the money. I am not sure that I would be comfortable routing all of my calls through their servers. I'm sure they will figure out a way to do voice recognition and show me ads based on the words I use on my calls - and, this is probably on the innocuous end. That said, I'm sure this would be a very useful service to have if you can deal with that.