The prize is 1000 pounds and it’s for writing an article about science. As far as I can tell, there are no residency requirements, i.e., you don’t have to be a resident of the UK to qualify for this competition [ETA April 8, 2011: Yes, you do have to be resident in the UK or Ireland according to my commenter and to GrrlScientist’s April 8, 2011 postiing] which is being held by Wellcome Trust, the Guardian, and The Observer. Here’s more from the April 7, 2011 article by Alok Jha on the Guardian website,

Our competition kicks off today to find talented new science writers.The Wellcome Trust Science Writing Prize 2011 in association with Guardian and The Observer aims to celebrate new voices in the ever-growing ecosystem of science journalism.
We want to identify some of the best writing about the remarkable ideas and stories emerging from the world’s laboratories, field trips and research journals. If you can enthuse people about cutting-edge particle physics or the latest developments in synthetic biology, this is the competition for you.

The deadline for entry is May 20, 2011. Here are some more details from Jha’s article,

… in summary, we’re looking for an 800-word article from non-professional writers suitable for publication on guardian.co.uk, in the Guardian or The Observer. There will be two prizes, one for professional scientists and another for everyone else.
The entries can be traditional newspaper features or writing suitable for the web that utilises the medium in an innovative and appropriate way. Bear in mind, however, that this prize is primarily about the writing and is not meant as a way of recognising expert programming skills or multimedia.

For an entry and more information you can go to the Wellcome Trust website.