From what I've seen of Fraser over the last couple of seasons I honestly don't think Bullock would oust him from the #1 jersey as easily as some might think, even in the short term. I also don't think ANY player should be guaranteed first team football and they need to prove they are the best available option.
We are in a decent situation at present whereby it looks like the Dunfermline are for the drop (who'd bet against it?), and I think that allows us the luxury of giving Fraser a bit of time to come up to speed, however, if we were to get the chance of a keeper who was unarguably better then we would be stupid to pass it up. At the end of the day Fraser is only 1 player, and as much as I like the guy and think he has what it takes, it's the team that comes first and in the SPL it's dangerous to try and nurse players too much on the park as the consequences of things going wrong are drastic.
The ball is squarely in Frasers half of the pitch, it's up to him to go out again on Sunday (and in each game after that) and prove he is worthy of the #1 shirt and I for one will be behind him 100% - I've seen what he is capable off and I know that if he continues to apply himself then there aren't many keepers out there (within ICT's reach) who are capable of taking that shirt off him.