Yea, but if you want to go to the center, it could take a while, especially if there is traffic. I'd set aside at least an hour to get to the airport from old town square if you want to be safe...that having been said, the bypass is now complete and I've heard good things about traffic being a bit lighter :)
You can also take the metro to Dejvicka on the green line and take a cab from there.
To order a taxi, I'd suggest AAA. Just send them and sms at 606 999 069...the format should be like this:
ORD YOURNAME * ADDRESS
ORD tony * dejvicak metro
they'll send you a confirmation.
Hope that helps