At the airport and tourist shops, you can buy prepackaged plants that have already been inspected by the Department of Agriculture - you can carry them right on home.

If you want to buy a plant somewhere else, you can go to the Department of Agriculture and have it inspected before shipment. We did this when we shipped plant arrangments to mainland friends for xmas last year. I was wary, but it was really easy, at least in Honolulu. There wasn't a soul at the Department of Agriculture desk - they were very friendly and it didn't take much time at all. Just be aware that there are some states (including CA and TX, as I recall) to which you're not allowed to take anything.