What you are looking for would be called a KVM switch that has a Mini-DisplayPort input.
As far as I can see, this is the only thing out there that fits the description.
http://www.atlona.com/Atlona-2x1-Mini-DisplayPort-KVM-Switch.html
If you note on the web page, it says that it is discontinued, but I would imagine that if it ever made it into production, that there would be some floating around. And you would still have to get an adapter cable to covert the output of your PC to the 'MDP in' ports on the KVM. A simple and cheap cable if you have a DisplayPort out on the PC, but would need more expensive and complex adapters if you don't.
I have never used or even seen this KVM product, so I have no idea how well it works. Or even if it was ever actually made.
If you have not gotten the monitor yet, I would instead get a higher end Dell 27 inch. Like this....
http://accessories.us.dell.com/sna/...2&lid=1838870&acd=pl_dhsea:028093,,901pdb6671
Besides usually being cheaper, it has multiple inputs built in, and you can switch between the inputs pretty easily from the front panel. Downsides: No Apple logo, and no Apple style. It also has a matte screen, which may be good or bad, depending on your needs. But heck, for the cost of the Apple 27 inch, you can often get a Dell 30 inch. (they have discounts and sales pretty regularly.)
Or if you have the desk space, for the cost of the KVM, etc, you could just get a second monitor.