We all know the phone companies cripple Palm’s ability to use a Wi-Fi adaptor. Of course, if you hack your Treo, you might be able to use the Palm’s battery-sucking Wi-Fi card, and lose your memory slot in the process.
Enfora makes a pretty slick and relatively cheap self powered Wi-Fi sled. While it makes my Treo 650 look even geekier, it's sure nice to be able to retrieve mail without using my data plan. I travel a lot, and it works really nice in other countries where data rates are outrageous but free or cheap Wi-Fi is readily available.
Another tip (which everyone probably already knows): Using PdaNET software, you can utilize a Treo and many other Smartphones as a data connection/modem for your laptop using your carrier's data network. On Cingular's Edge network, I get connection speeds of 384 Kpbs (not bad!). Using a Cingular 8525 on their 3G network, it’s 2.4 Mbps!