This is considered a normal and accepted practice, and there are many companies out there that do just this sort of thing. From the perpective of everyone else outside your AS everything will be perfectly fine. The only thing you'll need to be aware of is that your islands will not be able to see each others routes due to AS Path loop prevention mechanisms inherent in BGP, since they will see routes received with your AS in the AS Path and will assume some type of loop is taking place. You can get around this by configuring AS loop support on your routers.
My apologies for the top-posting, I'm using my BB.
Stefan Fouant
Neustar, Inc. / Principal Engineer
46000 Center Oak Plaza Sterling, VA 20166
Office: +1.571.434.5656 
Mobile: +1.202.210.2075 GPG ID: 0xB5E3803D stefan.fouant@neustar.biz