I have to agree with one of the earlier posts. A 10/100 Ethernet connection would probably be one of the best things that could be done. It is cheap and easy to implement. Of course, you would have some support issues as sales increase, you will get less technical users... Therefore, the product must also be bullet and idiot proof.
The second best thing would be a wireless option. There are several USB based wireless solutions. Although they are certainly more expensive than ethernet, they would allow the unit to be programmed from several hundred feet away... ie, in the garage. USB is about the same speed as the low end wireless solutions. There are a number of new - Home networking - products on the market that are geared to the extremely price sensitive users. The nice thing is that since it is USB and the wireless is a transparent bridge, you can use any of your existing software and custom solutions would not be difficult.
For me, this would be the most convenient as I could schedule a my playlist updates and they could be done at night or in the background.
Wireless would also overcome one major problem with the unit. No matter how good your connectors are, continous insert/remove operations will cause a failure of the contacts or atleast less than perfect connections. I would be far better to avoid this problem as it will most certainly show up in the warantee department. Repairs will also increase as user knowledge decreases. Take it from me, after 15 years of computer engineering and 4 years as an ISP, in order for an item to be a successful consumer product, it must last atleast 5 years under the most grueling conditions and will be handled like a frisbee.