I will perhaps begin my answer, since the GST arrangement has come up a number of times, by drawing a distinction between that and what is proposed here. Under the administration of the GST, the administration is a harmonized system. There is an agreement between the two levels of government to keep the system harmonized. It is one system, essentially, with two parallel sets of rules that are virtually identical.
As has been alluded to a number of times, there are differences in policy and law between the federal income tax system and the provincial income tax system. An example came up earlier in the discussion with child care expenses. Quebec has made policy choices to have a subsidized system and therefore does not provide a deduction for child care expenses. It provides a subsidy and a credit under certain circumstances. There are very important differences at the core of the system.
I'm not sure if it's a question of drifting apart, because they are not together now, and the discussion really isn't about harmonizing the two systems, because they reflect different, very important policy choices by the federal government over time and by the provincial government over time. It's not necessarily a question of drifting apart, as they are already apart, and that really goes to the issue of a single tax return and the economies in compliance that can be achieved when you are dealing with two different systems.
I hope that goes some way toward answering the question.