Special Occasions
Rule: It's your party, you can buy if you want to - but you shouldn't have to.
The below applies equally to birthdays, bucks/hens nights or Nobel prize nomination celebrations; any sort of special occasion where you are the centre of attention.
It's the role of fellow drinkers to make sure that the buck/hen, birthday girl/boy or laureate doesn't go to the bar. However, in a group of less-familiar drinkers, over a fairly lenghtly drinking session, the other members of the group are not obliged to continue buying drinks for the special guest. Ideally in these situations each member of the round should buy one round for said special guest, but after that everyone is returned to equal status and normal round etiquette applies.
Note that this is the opposite for our friends across two ponds in the UK, where the special guest tends to be the one who buys the drinks.