this might be naive from the point of view of the parallels developers, but it seems to me that much could be fixed, and at the very least many of these usb problems could be debugged more accurately if one major part of the equation was removed --- the conflict with osx trying to take control of the usb device and not allowing parallels to use the device as a result. If there could be a way to "turn off" usb access on the mac side for a particular usb device, and this could be controlled on a device-by-device basis within a popout control panel in the parallels main screen (config screen, not within the guest os), and if the list was saved automatically whenever a change was made (like if I told the list to give parallels control of my ipod then the next time I start parallels it takes control of my ipod if it's connected or when it's next connected), that would be really nice.
This would solve many of the conflicts we're all experiencing right now, and I personally have managed to work around some of those conflicts on a device by device basis by disabling the usb device on the osx side when needed but this isn't consistent or easy at all (it's a pain in the butt actually). But it would allow more devices to work, it would allow us to say exactly which devices run under parallels or osx (since it can't run under both at the same time obviously), and it would give us the ability to send more useful bug reports to parallels about which devices honeslty don't work.
As an example of how I'd do it, make a scrollable list of usb devices that are currently connected to your mac with a checkbox beside each saying "use in parallels". if it's unchecked it's useable in osx, if checked it'll be disabled in osx and useable in parallels' guest os. Due to the nature of usb devices being plug and play, this list will have to save itself with updates every time a new usb device is seen when parallels is running, and then when you next run parallels even if your ipod or whatever isn't connected it will still be in the list if it was used in the past with parallels and that way you if you have a device that you always (or almost always) use with parallels then it will remember that setting in the future when you plug it in again.
This preference setting would only take control of osx while parallels is running, when parallels isn't running the particular guest os in question then osx will try to use all attached usb devices as usual.
Of course this is just a suggestion of how parallels could do it, that's how I would do it if I was coding this, but it's a quick thought - no real research put into it, and I don't know if it's possible at all.
Wouldn't it be nice though?
Cheers,
DKelley