I was pre-approved for a mortgage in 2011 while I had I think a month or so left in my 3 month probationary period by First National Mortgage. Managed to get one of the lowest rates and below posted ones as well. I worked with a mortgage broker to get the pre-approval and she mentioned while it might limit the options being on probation (some banks won't touch you), if I could prove that I was working in a line of work which I have in the previous couple of years at a salary that was consistent with previous jobs, some places will accept that. Essentially, if you are a career IT support guy who makes $60K a year and your new job is still an IT Support guy making close to that, you should be okay. If you are a career IT Support guy and you are working now as a Registered Nurse for $40K a year, they might have some hesitations.
Also factoring in of course would be your credit history. If you have perfect or near perfect credit history, they would be more inclined to be lenient on length of job. If you have some issues, they will probably require you to have more time in so that you are less of a credit risk.
The broker I used was Pat Hampton (now Forgio apparently) and she specializes in first time mortgages and was great for asking questions. Definitely would recommend her.
http://www.qmortgage.ca/team/name/pat-hampton