I know that iriver has refused service on devices that show evidence of Rockbox being installed on them, but the iriver devices supported by Rockbox are all discontinued, and (I believe) most users are inclined to service them by themselves.
Archos, from what I've read on these forums, won't service devices flashed with Rockbox. Again, these are such old devices that I don't know what their stance is today.
Apple is one that is a bit of a question mark. Some people managed to get their devices serviced, and some were turned away. On my end, I tested this by taking my iPod video to the Apple Store in New York and asked for a new battery. The minute the guy turned my iPod on, he handed it back and told me I have voided my warranty by installing Rockbox, which, as he put it, is "unauthorized software". So yes, I did waste $59 for a 3-year protection plan from Apple that's about as good as toilet paper now.
I don't know what SanDisk's stance on Rockbox installation is, but maybe someone else with Sansa experience can answer that.