hi zikke,

ok, it's hard to tell exactly, but from you post, i am guessing that you are in a city in the "developing world" which means it is likely that:
a) gas is VERY cheap
b) the "environmental friendly" aspect as some suggest is pretty much irrelevant

so basically unless you can find some "interesting" aspect to promote, it basically just comes down to economics:
* are the costs of buying, maintaining, insuring a motocycle and paying any motor vehicle traffic citations significanty MORE than the costs for a bicycle? (for example, in NYC i think the answer would be YES)
* are there any signficant "no motor vehicle" areas where a cyclist would have better access and thus be FASTER than a motorcylce?

i think if one of both of these points is possible you might be able to start with a combination of motorcycle and bike messengers (soas never to offer LESS service than the competion by serving the "out-of-town" areas) and then try and build on the advantages of bike messengers (e.g. theoretically you could employ a wider range of people as the "requirement" to have a bicycle is less than that of a motorcylce so maybe you can pay less?? although you still need people competent and dependable enough to handle the product)

--> but basically i agree with Peter: if you can offer the same dependable service faster or for less money than it is pretty much irrelevant if it is a "sweaty guy on a bike"