Seriously? You're hesitating over this decision? Then let me take this opportunity to salute you. You're good.
We have provided a handy flowchart that might help you resolve this dilemma. (Warning: do not give the flowchart a half-truth. It gets cranky when you do that.)
Should you eat that cupcake? Click on the flowchart to determine the answer:

Obligatory disclaimers:
There are 181 calories in the average Hostess cupcake.
An adult who weighs 150 pounds with an average metabolism would need to exercise for half an hour jumping rope, kick boxing, or cross country skiing, to burn off two average cupcakes.
More serious disclaimer stuff:
The flowchart is directed toward people who, like St. Augustine, want to be good --- but not just yet . It does not apply to people with serious eating disorders, who might find one of the following sites a lot more helpful:
A couple of great blogs:
The Great Fitness Experiment
Scale Junkie
And some professional organizations:
NEDA (National Eating Disorders Assn)
ANAD (Assoc. of Anorexia Nervosa and other Asssociated Disorders)
Something Fishy: a great resource for everything ED. Forums, treatment finder, etc.
USF Hope House for Eating Disorders
(Thanks to Charlotte and Diana for their link recommendations.)