I've been in this situation, someone's birthday or something, and the cakes come out. I would stand around the cakes with everyone else and lie though my teeth, oohing nad aahin, "Mmm, those cakes look great" etc, then just make something up about them not really agreeing with me. Theat usually satisfied them.
Very interesting point. I had wondered about the possible offence of not accepting food. Your point about trust seems spot on. But why they should actually be offended beats me - it's just a cake. With all this stuff in the media about poor diets, clogged arteries and obesity, you'd think they'd have accepted that not everyone is going to partake everyt ime, especially if, as you said, it happens every day.
Now my beef with all this is when I was asked to go and get the cakes: our supervisor at one particular job, who was a great bloke, would come up to one of us, shove a fiver into our hand an ask them to go down the road to the cake shop to get some dooughnuts. I dreaded it, feeling nervous about going into shops as I do. The day came, and I blew it, getting too many cakes. Oh well!