Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
For those still interested in this method of getting to/from Ithaca, it appears that Megabus starts selling seats on the Short Line runs between NYC and Binghamton exactly six weeks in advance of travel date. (Note that Megabus has raised its ticketing fee from $.50 to $1.50.)
Another option to consider -- though not as cheap as a trip with the cheapest Megabus fares -- is Greyhound's Internet fare between NYC and Ithaca. Looks like a fare of ~$32 one way is offered. And Greyhound's 2:45 PM departure from NYC's Port Authority Bus Terminal to Ithaca's State Street Terminal takes about 20 minutes less time than the typical Short Line trip. But, unlike Short Line, Greyhound does not offer any stops on the Cornell campus.