Okey dokey, can someone explain to me how to blur the background of a photo using Adobe photoshop cs3. My rugby team are having a picture in the paper to promote the sponsor of our training jackets and we would like the background blurred so it's nice to learn!

to be honest, it's not worth the effort for a simple photo for the paper. to do it properly you'd need to totally isolate the background so that it can be correctly gradient blurred without the blurring of the players interfering with the blur effect.  the rest is as roo has suggested above.

the best way would've been to have the photo shot with the correct aperture in the first place (yeah yeah i know, easy to say AFTER the event), although having a blurry background or not won't make a great deal of difference to the sponsor or the casual punter looking over the photo.

a poorly done retrospective blur is worse than no blur at all.

my intention wasn't to be harsh. based on experience, it's not worth the hassle that's involved to get a technically perfect job. the sponsor's main concern is getting the publicity rather than having a nicely blurred background. if you do really want to go to the hassle then this is the method i use....

take base layer, duplicate.
create a very accurate selection around the main subject (painted mask is easiest). delete this from the 'background' layer so you're left with a background image without the players in it. masking will not work. you need to delete.
before getting rid of the selection, use it to create a mask over the 'foreground' layer, thus creating a layer than only has the players visible in it. now turn this 'foreground' layer off.
swap back to the 'background' layer and clone all around the edges to partially fill in the void left by the main subject after it was removed.
apply the necessary blur - the cloning will help avoid poor blurring at the edges where the players were. to do this properly you'll need to vary the blur across the photo to match depth of field - grass at their feet will be in focus, steadily increasing in blur the closer to the camera and the further from the camera the distance becomes.
when that's complete, swap back to the foreground layer and fine tune the mask so that no part of the background is inadvertently shown.

as i said, not worth the hassle for a simple publicity shot, as to be perfectly honest, no-one will really notice the background...why?
team shots are typically done with wide angle glass which has a high depth of field. to suddenly see a blurred background goes against virtually all other team photos you'll ever see and will look wrong. only my opinion though.