According to this article Kick-Ass is to be released in the USA on April 16th 2010.
There are no release dates as yet for the UK, Europe or the rest of the world.

http://www.thehdroom.com/news/Lionsgate_Dates_Kick-Ass_Delays_The_Expendables/5668

Lionsgate Dates Kick-Ass; Delays The Expendables

September 02, 2009

Lionsgate has chosen April 16, 2010 as the date to unleash Mark Millar's Kick-Ass starring Nicolas Cage and Aaron Johnson into theaters, bumping The Expendables in the process to clear calendar space.

Kick-Ass was picked up by Lionsgate shortly after it erupted onto the scene at Comic-Con and supposedly began a bidding war between multiple studios for the distribution rights. Adding completed Kick-Ass to a portfolio that already included Sylvester Stallone's The Expendables instantly gave Lionsgate a one-two action punch for 2010.

Per Variety, the recent success of "male" films in August including District 9, Inglourious Basterds, G.I. Joe: The Rise of Cobra and The Final Destination gave Lionsgate enough confidence in the month to move The Expendables from April 23 to August 20.

Going up against Kick-Ass on a now testosterone packed April 16 will be Death at a Funeral with Chris Rock and Tracy Morgan, Piranha 3-D, and the Saturday Night Live sketch-turned-movie MacGruber.