Wheel weights (they fall off especially if they have been on a long time, or if they have been balanced incorrectly in the first place)
Tyres out of round (happens a fair bit with cheaper brands)
Wheels out of round (happens more than you might think, especially if you live somewhere with lots of potholes)
Excessive play in swivels/ball joints/track rod ends, or whatever you want to call them (usually only on older cars)
Excessive play in rack bushes (usually on older cars)
Front wishbone subframe bushes worn (this is very common and has been seen on cars as young as 25k miles!)
Depending where you are and your technical ability, its might be worth getting it checked out by a MINI place such as ThinkMINI (worcestershire), 1320 (near bedford) etc etc if it does not turn out to be the wieghts or the tyres or something simple like that.