I think I have to go with the majority of evidence presented here and side with Jaffa Cakes being cakes, for the following reasons:
1. As Pete Hosk states, HM Revenue & Customs class Jaffa Cakes as cakes for tax purposes.
2. They are (initially at least) soft like cakes.
3. Their name contains the word 'cake'.
Admittedly they inhabit the biscuit aisle in most leading supermarkets, but if McVities want to position Jaffa Cakes in the biscuit aisle to maximise sales then that, as Bobby Brown would no doubt say, is their prerogative.
This thread may have veered slightly away from its original subject matter...