I came across this on the government website, not sure if it has been implemented yet but it looks like they want to crack down on deliberate overcontributions. 100% tax seems to be a good way to discourage this.

Deliberate Overcontributions

The TFSA rules allow a holder to contribute, for 2009, a maximum of $5,000. Contributions and associated earnings may accrue tax-free in the TFSA and may be withdrawn at any time without any adverse tax consequences. Contributions in excess of the contribution limit are subject to a tax of 1% per month on the highest amount of excess contributions for the month. This tax is generally sufficient to neutralize the tax benefit of overcontributions. The Government of Canada has become aware that in certain situations, and subject to the existing anti-avoidance rules in the Income Tax Act, some TFSA holders are attempting to generate a rate of return on deliberate overcontributions over a short period of time sufficient to outweigh the cost of the 1% tax. On its introduction, it was not anticipated that the TFSA would be subject to this type of deliberate overcontribution.

Under the proposed amendments, any income reasonably attributable to deliberate overcontributions will be made subject to the existing advantage rules (as described above) and taxed accordingly. Pursuant to the advantage rules, the tax payable on the income will be 100%.

The Minister of National Revenue will maintain the discretion to waive or cancel all or part of the tax payable and the authority to adjust the taxpayer's TFSA contribution room accordingly in appropriate circumstances.

Link to the whole article

http://www.fin.gc.ca/n08/09-099-eng.asp