I know it's not Linux but I've got a project on in work at the moment - I'm going to be using a Palm based PDA. The one I'm using is a Tungsten C (or it could be W - I always get them mixed up) it's got WiFi built in and an SDIO slot which a barcode scanner from www.socketcom.com should fit into nicely, as soon as it's available!

I take it you've already seen their products though?