You could just ignore the notifications. On my hero I only see those if I drag the top bar down. Other wise I just ignore them. If you choose to mount then your phone will charge and work just like a USB stick (if you have a microSD card in it of course).
I have a whole box full of these cables and use them interchangeably on many devices. Camera's, Playstation 3 remotes,Nintendo DS, other phones, etc etc. I keep two in my back pack, two in my car, the there is three downstairs that are used for charging the PS3 remotes, or my phone. My experiment with the Pre was so painful because of it's use of microUSB, I only had one cable and it was a hugh pain in the ass.
You can find a cheap miniUSB car charger just about anywhere these days too.
It's important to note that the port on the phone and the end of the cable that comes the phone have a slanted shape that looks different then other miniUSB cables. This has existed on HTC phones for a while but it's fine to go ahead and plug in a regular miniUSB cable.