I heard June 1st also, but it may have been implied because the USCIS are doing a 60 day review (From February 1st) at which point the USCIS will draft a final proposal and give 60 days for review.
All this is listed on the PDFs on the USCIS website:
Q&A: Building an immigration service for the 21st CenturyQUOTE
Q. When are the new fees effective?
A. A proposed rule on the fee adjustments will be published in the Federal Register on February 1, 2007. The proposed rule provides for a 60-day public comment period. After receipt and analysis of the comments, USCIS will draft a final rule reflecting the public input. It is important to note that a proposed rule does not and cannot by itself, raise any immigration benefit application fees. Publication is only the beginning of the regulatory process where an agency announces its intentions to change its current regulations, and solicits public comments on the effect of these changes.
Statement of Director of USCIS regarding proposal to adjust fee scheduleQUOTE
At the outset, I want to emphasize that USCIS published the regulation as a proposed rule with a public comment period of 60 days. This means that no fee changes will go into effect until USCIS receives and analyzes comments and suggestions from the public, and I can assure you that USCIS is actively seeking feedback to the proposal. After analysis of those comments, USCIS will draft and publish a final rule reflecting the public input and will provide an additional 60 days before implementation of any new fee schedule. While USCIS loses $3 million every day under the current fee schedule, it is important that the public have an opportunity to weigh in on the proposal before any changes are made.