I have not been able to find any authoritative information on this topic (just some speculation that hasn't been very credible). There is nothing in the language of the act itself to suggest how it applies in transitional cases (those in various stages of become citizens).

In any case, the answer will have to depend on which PART of the Act one is referring to. For example:

1. Take the clauses that have to do with criminal conduct (stripping dual citizens of their Canadian citizenship in cases of treason) and increased penalties for fraud. This would presumably have nothing to do with when you applied for citizenship--going forward the new laws would apply to everyone, though it MAY be a function of when the act was committed.

But to take another example that will be more of a real concern for people:

2. What stage does your citizenship processing have to be at for the new residency rules to apply? From most stringent to least stringent, this could be:
a. Only if your application has already been approved and you're only pending the oath?
b. Only if it has been accepted for processing?
c. Only if it has been received (or possibly postmarked) before the effective date?
d. Only if you were eligible to apply for citizenship before the effective date?
e. Only if you have landed status before the effective date?

My guess would be (b) or (c) but then that's based on common sense and that's hardly the criterion that CIC or the legislature uses. Does someone have an authoritative response to that? (In another thread, someone suggested that the answer was (e) but I highly doubt that--there would not be all the hue and cry from existing PRs waiting for citizenship if that were the case.)

And for those doing the simplistic calculation of:
"OLD RULES: 3 years residency + 2 years processing = 5 years
NEW RULES: 4 years residency + 1 year processing = 5 years
=> It's all the same for me"
think again. The 1 year processing is not part of the Act at all (it couldn't be)--it's just a claim that the "simplified procedure" will lead to quicker turnaround, as one of the ways to get support for the act. This may or may not be true (having the decision making be all in the hands of CIC/Ministry "1-step process" without intervention by a citizenship judge "3-step process" can certainly speed things up, at the cost of reduction of due process and more concentration of power for the ministry, which was no doubt one of the hidden objectives).

I did, subsequent to my post, find this quote at http://o.canada.com/news/national/new-citizenship-bill-will-make-it-harder-to-become-canadian/

"If and when the bill passes, the new streamlining rules will apply retroactively to those in the application backlog but they will not be subject to the new residency requirements, Alexander said."

which is a partial answer, to the extent that second-hand verbal statements by politicians can be believed, that (b) and later applicants will get the "new streamlining [sic] rules" and that (b) and earlier will not be subject to the "new residency requirements," so the (b)'s may be getting the best of both worlds. It also IMPLIES that those later than (c) WILL be subject to the new residency requirements (with those in (c) currently in a sort of limbo).

So I would strongly suggest that those eligible to apply under the current rules who have not done so cancel their other weekend plans and put together a complete application and send it in on Monday if not earlier, to maximize the chance of it being solidly in (b) before the new Act takes effect.

Am still looking for more definitive information on this, so please do post updates if you find anything more on this, or if you are or have access to an immigration or citizenship attorney who can give a reasoned prediction on how the retroactive part of this will play out.