I've got a C6-00 (firmware v11.0.029). The internet radio application on the phone is pretty much useless if the slider is closed, and especially bad if you background the application.
Useless in the sense that it plays for a few seconds, pauses (as if buffering) and resumes. This is not a bandwidth issue because my link is 10mbit (when using wi-fi), and HSDPA (3.6mbit when using GSM).
I think the problem is caused by aggressive power management, that is, the Wifi radio goes into power saving mode too soon, causing the buffer fill to start a little too late for smooth playback, and perhaps the power management is less aggressive when the keyboard is slid out.
Any one else experiencing this problem? Am I right in my hunch? How does one solve it? Or how does one get Nokia to solve it?