I too came across a source indicating April 17 as the launch date for the 7990. Other sources (e.g. http://www.cpu-world.com/news_2012/2012032002_Radeon_HD_7990_Specs_Detailed.html
) points to a launch in April.
The price drop of current GPU's is with no doubt paving the way for the 7990.
I have searched exclusively through many Asian, American, and European sites using Google ("Radeon 7990 site:.tw" for Taiwanese site results only), limiting publications to the last hour, 24 hours, and week options. There is nothing more precise than March, April, end Q1 or beginning Q2.