Are you going to be in an area with cellphone coverage?
If so, you could use a cellphone like a Motorola RAZR as a wireless modem. The GSM version T-Mobile and Cingular sell in the US will work in Europe--the Verizon and Sprint versions will not.
The T-Mobile plan I have is $40 per month for lots of voice minutes plus $20 per month for unlimited data transmission. That's for use in the US. Data rate is comparable to dialup, not high speed. In Europe, T-Mobile charges about $1 per minute for voice and something per kilobyte for data.
I don't know whether this will work with a powerbook. My experience is with a Windows laptop. Windows recognizes the cellphone as a modem.
If you only needed voice, then people here will tell you to buy an unlocked GSM phone on eBay and buy a prepaid SIM when you get to Europe. But I doubt that you can use a prepaid SIM for data service.