Unlimited or not, the cell companies do not wan't anyone downloading "massive" amounts of data over their network. And I say it was smart of Slacker to not allow caching over the cell network. Besides it would bog down the network if there are several people downloading all at the same time on the same tower. Although they SHOULD allow caching over Blackberrys that have wifi capability.
Yep I realize that unlimited doesn't really mean unlimited most of the time but I think it should still be an option for when you're in an area that you don't have access to wifi or a PC (or have a curve like mine that can't use wifi). Throw up the massive warnings that all other apps do when you attempt to do it for CYA, and maybe don't allow it on carriers with known problems with people using data like AT&T. More options are always better though.
One thing that did kinda cross my mind though last night was let's say someone goes to Canada, and has cached stations. Does the slacker app fart out when you start it and sees the Canadian network, or does it let you listen to cached stations and just not stream? I might be going sometime next year and I kinda wanna know (phone has a setting to prevent data roaming so I'm not worried about that, I'm just kinda worried about the app seeing a Rogers or Telus network ID and puking all over itself)