There is nothing about a new timing belt install that should affect the idle speed. Even if the old belt was drastically stretched......
Unless something was done wrong......a vacuum line/hose knocked loose or split would be the most likely suspect. Getting the belt one tooth off might do it.....hard to say. Take it back to whoever did it and complain.
Tom's right to clear the Keep alive memory after doing the timing belt.
I just replaced mine and hooked the positive and negative leeds together while doing the work. Got it all done and it idles like a champ (700 on the dot).