I wonder if they tell us this stuff just to make us go away. Last year's PhD letters were dated April 25th; I see no reason as to why it would be different this year. Our grad studies office is offering a "how to prepare for Sept 2009 SSHRC/NSERC apps" on May 5th; I suspect that this means that we will know by then.
... or it could be because that's when the OGS letters are mailed, and she's assuming that it's around the same date for SSHRC. (OGS, by the way, is supposed to be updating their website with results on Saturday, according to my dept secretary.)
Last year's winners received a letter dated April 25th (a Saturday this year). I'm not counting on hearing until the last week of April. Your department may get the results before you receive the letter, so if you're in good with your department secretary, it might be worth speaking to him or her on the 24th ... just in case!