I worked in transportation planning in the Harrisburg area for almost 8 years until mid-March 2008. I also have a lot of interest in public transit. Though I never used it, I heard mostly positive comments about the RabbitExpress service during the time I lived in Harrisburg. I do know they stop at the park-and-ride next to I-83 Exit 33. Rabbit Transit in York had to expand the service a few months after they started it (which I believe was in about 2006) due to its popularity. You may want to contact Rabbit Transit for more information.
If the RabbitExpress wouldn't work for you, an alternative would be to drive to the shopping center near Capital City Airport. Capital Area Transit (CAT), the transit provider in the Harrisburg area, has a bus route (I believe Route A) that stops at a park-and-ride at that shopping center (which is anchored by a Giant) and goes through New Cumberland and part of Lemoyne before going to downtown Harrisburg. Check out CAT's website (at
www.cattransit.com) for more information.