Yes. Apple makes an AirPort Extreme base station with a built-in 56K modem for exactly this purpose. Plug the base station into the phone line, set it up with your ISP's info and phone number, and your wife can connect wirelessly from her laptop.
If your Windows PC has an 802.11 card, you can both use the connection at the same time. (Though it might be kind of slow to do this with a dial-up modem.)
The bad news: The base station with modem is $250. (The 'Book will also need an AirPort card; all new PowerBooks come with them, as does the top-of-the-line iBook; otherwise, it's another $100)
(Usually, Apple's wireless products are much more expensive than those from Linksys, DLink, and other companies. I looked for, but couldn't find, an 802.11 base station with a modem from another company. It seems Apple's AirPort base stations are the only ones that have a modem. Anyone know of any others?)
For more info:
http://www.apple.com/airport/