juicychops - most celebrities DO get paid expenses when they do charity work if not an actual salary. They are not expected to pay for their own accomodation, flights etc. I heard that Bob Hope was notorious for this when he played at charity golf events - a game he loved anyway so he treated it as a freebie holiday with lavish all expenses paid. Not quite what most people would think was a 'charitable gift' of your time.
Also all the costs of organising and runnng a charity event are obviously deducted from the proceeds and lot of that is people getting paid a salary who actually work for the charity. All the services services they buy in obviously get charged for as well so someone is making money there.
Charity is big business - and I have done consulting work for a few national charities so I know what they are really like behind the scenes. Just like any other commercial organisation, out to maximise revenue at lowest cost. Its just they have no shareholders, only beneficiaries.
That said, I feel sort of YABU because I would not feel comfortable with say a friend I sponsored taking part of the sponsorship money for any reason. I would want it all to go to the charity. I knew someone that effectively funded a 'jolly' with mates to travel across the Saharah Desert and dressed it up as 'fund raiser' for cancer relief but in reality very little went to the charity. It was just him and his fairly wealthy friends getting a kick. I refused to donate.
I guess my feeling is that if you like running in events then you should pay for that yourself and if you want to raise some money while you are doing it then that is a separate issue. I would not use the fact that you are raising some money for charity as a 'cover' for getting some free funding of a hobby you enjoy doing anyway.
Perhaps the only really ethical way round this is to explicitly tell people up front that you wil take say £50 for entry costs out of the sponsorship money and then if your sponsors willingly agree to it then I do feel that is OK.
Hope you feel that is a balanced answer to what is actually a very tricky ethical question.