You won't be able to see any new competition until the end of 2008 at the least. The spectrum auction is in May and will probably go on until June. Then new competitors need negotiate with the big 3 telcos on the use of their cell towers and other agreements relating to coverage. Then they need to set up stores or websites and price out plans, etc...
In the short term, I would wait until the iPhone is released because:
1. You might like it
2. If you are waiting for good data plans, the iPhone might force the other telcos to reduce their rates.
3. More EVDO or HSDPA phones will be released.
I've been hearing rumors that the iPhone will be out in Dec or Jan.
Also, if you sign a contract, you will pay system access fees. If you stay prepaid, you don't pay these fees.... at least that is what a Rogers CSR told me.