The beginning of October is early for a repositioning cruise from the Med to the Caribbean. They are usually the end of Oct. that is why you cant find one. Unless it is going from cold areas to warmer like alaska to southern states or to the far east. I have found this one which includes the date you wanted on Holland Americas The Ship: Zuiderdam.
The Trip: Vancouver to Ft. Lauderdale -- 18-night Panama Canal
Departs: September 26, 2009
Itinerary: Vancouver, Los Angeles, Cabo San Lucas, Acapulco, Huatulco, Puerto Quetzal, Corinto, Panama Canal cruising, San Blas Islands, Santa Marta, Half Moon Cay, Ft. Lauderdale
Otherwise there is a P&O cruise on Oriana on the 30th Sept for 22nights round trip from Southampton to 6 islands in the caribbean and back to Southampton via Vigo and Madera. Lovely ship and lovely cruise - I have been on it myself !! Also try Princess repositioning from Alaska in Sept to far east and Japan. It also depends on how long you can go for and the amount you have to spend ! Happy cruising.
As Macro has advised your too early for the main exodus of ships back to the Caribbean. The only two I found in your date frame were HAL Noordam out of Rome on the 6th Oct 26 nights Insides starting about £1800 & Seabourn Odyssesy 3rd Oct 38 nights Balcony's starting around £16000 (yes I did get the noughts correct). If you look at November there are some cracking deals on RCI Navigator & Independence repo'ing back across the pond 12/13 nights from around 500 quid!!!....Regards Wilba.
Mrs M and Wilba have given you the main answers. Having done a good search I came up with nothing better. Last year it would have been a different story and 2010 could be so again but that doesn't help you. If you have set as your remit the stipulated restrictions then there is nothing else, change any one of those and it could be a whole different kettle of fish. We would all love to help someone new to cruising because we love it ourselves so if you are able to change one aspect of your request then write again. Regards .......Neil.