I can confirm these claims. My system is a Dell Latitude D620, Core2Duo processor. Any time the DPH-50U is connected to the system (and the driver is installed), my CPU usage is a constant 30%.
I used Sysinternals process explorer (
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx) to isolate the source of the CPU usage to a DLL file that is part of the DPH-50U driver package. It looks like this DLL is CONSTANTLY generating CPU interrupts. Likely this is an extremely badly designed piece of software that simply polls the device hundreds of times per second, even when you are not on a call.
Given that this device went end of life in August of 2009, I don't expect any help from D-Link (not that they ever offered any help with this device prior to 8/09).
FYI to anyone else who has been fighting with this device: The same device is OEM'd by D-Link, US Robotics, Atcom, and perhaps a few other companies. The actual chip maker has no end user facing presence whatsoever. This is why ALL of the drivers have the same problems, because they come from the exact same source (the original chip maker). The different OEMs seem to offer different levels of release of the original source driver, but they are all problematic in one way or another.
Too bad nobody seems to make a USB to RJ11 adapter for Skype that "just works".