I've got a Centro, with the Sprint Data Plan. Can I use the phone as a wireless modem at my lake home, and eliminate the broadband I'm paying for down there? I've seen an ad for software that is supposed to allow that. What's required? What kind of speed should I expect? Is it worth messing with?

Can anybody give me advice?

I just took a quick look at the Centro on Sprint's site and they do show it as Phone As Modem compatible. So, either you can put a Sprint PAM plan on your account for whatever it is they charge for it, or you can use third-party programs like PDANet from JuneFabrics (Palm version) to use your phone in this way (possible technical violation of your Terms Of Service). I believe the USB cable required to tether it to your computer is provided with your phone. You can find more info on this in several threads, forums, and Sprint's web site.

Speeds will depend on many factors, such a proximity to towers, roaming issues, environmental issues, and so forth. But usually they run around 700 kbs to about 1.4 MBbs download, 400 kbs upload.

I tethered my phon to my laptop with verizon and I feel it was well worth "messing with". But your performance will not be as fast as broadband which I assume is around 2 Mbs down 400 Mbs up or thereabouts.