I have never heard of aspirin acting alone for fat loss. However, as ironman mentioned, it has been used in an ECA stack (ephedra, caffeine, aspirin-in fthe white willow bark form). The ephedra and caffeine are CNS stimulants (central nervous system), and work in conjunction with aspirin's blood thinning properties to cause the body to raise it's core temperature slightly, thereby (supposedly) raising the body's ability to burn more calories at rest. Also, due to the CNS stimulation thing, apetite suppression usually occurs along with it
I'm not sure what's up with ephedra these days, it got banned a few years back, then I heard the natural form was re-instated, but not sure about that. Most diet pills these days are basically caffeine with a white willow bark derivitive (form of aspirin) with some other added ingredients to "free up fat for metabolism" (usually things like l-carnitine, etc). These things do somewhat dull the apetite, and will get you wired like drinking multiple cups of coffee, but I have doubts as to their overall effectiveness. If you choose to try it, start on the low end, or you may find yourself staying awake all night.