While I was Googling to track down one of my own stories, I came across this BlackBerryCool.com entry promising that Mac users will soon be able to use their BlackBerry handsets for tethered Internet access.
I've been tethering a MacBook Pro through my T-Mobile BlackBerry 8800 since May; I point to the very well-documented procedure in a prior entry to Enterprise Mac. Follow the instructions, and your Mac will dial your Bluetooth-equipped BlackBerry like a modem and link it to your wireless operator's data network--in T-Mobile's case, that's 128 Kbps EDGE--whenever it needs access to the Internet. Tethering isn't unique to BlackBerry. I've done the same with Symbian (e.g. Nokia, Samsung) handsets, and tethering is baked into Windows Mobile. Whether you can use it, and how much it'll cost you, is really up to your wireless operator. Free tethering is permitted under T-Mobile's BlackBerry Unlimited plan, while AT&T requires the use of a special data plan. I'm not familiar with other carriers' policies.
iPhone does not yet support tethering.