Because of the location that you describe, it sounds like an overflow hole, but that is unusual on a kitchen sink. The sink must be porclein rather than stainlesss steel?
When the sink is turned over can you see through the hole? If you can't then it sounds like an overflow. If's it's an overflow that shouldn't interfere with the installation of a disposal.
If the sink is stainless and you can see clear through the hole, then I'm guessing that the hole is either for a spray or for a bottle that holes dishwasher detergent or hand chream. If you don't have either, the plumber or you will put in a chrome plug from HD ...cost about $2.00.
I'll be curious to find out what it is because I'm not sure I've seen a hole just a little lower than the faucet deck.

I bought it from a vendor in Canada. It is a stainless steel sink, and the hole (actually its parallel slots) are definitly not for soap/sprayer.

Heres a picture of the sink:
http://cgi.ebay.com/BRANDNEW-STAINLESS-STEEL-DROP-IN-KITCHEN-SINK-8501_W0QQitemZ350002098333QQihZ022QQcategoryZ20603QQcmdZViewItem

The slots are right under the faucet deck. I think it IS an overflow, but with a flexible tube that connects it with the regular drain. Darn Canadians. Getting me so confused!, LOL!