Conard - 8:12 pm on May 11, 2009 (gmt 0)
I do the same type of thing as you with the wireless terminal and trade shows.
We keep our paper receipts for about 6 months.
There have been a few times when a customer has received their statement and seen our business name and town and tried to file a chargeback based on the fact that they have never been to our town or area.
A copy of the signed receipt along with a note of where we were during the sale has gotten the chargeback reversed every single time.
I think the longest time frame we have had from sale to dispute was around 4 months but we keep the receipts a bit longer, just in case:)