So, a friend of mine wrote to Rob Prichard a few weeks back and recently shared the letter and response w/me.
Basically, he wrote asking why there was no 30-min service on Lakeshore yet, as well as the other long proposed service improvements on other routes.
He got a brief answer from Prichard of the 'thanks for your input' variety; but also got a follow-up letter/email from Gary McNeil, GO's GM.
GM said basically that in regards to more rush hour trains, they lack enough locomotives right now. That new ones are arriving all the time, but through then end of 2010 are being used as replacements rather than fleet add-ons. He suggested net new locos were due in early 2011, and rush hour service add-ons would follow.
As to off-peak service, he basically said this loses money (ie. requires subsidy) and the province has been requested to provide this in fiscal 2010-2011 and if that money is forthcoming, the service will be to.
So for those waiting, time to bug the higher-ups, see Kathleen Wynn in transport, and the Premier if you want more GO service.