100% of one core seems excessive. mdworker is the metadata server process. You'll find it taking a fair amount of CPU power whenever you're either modifying files or mounting volumes.
Unless spotlight is still indexing your drives, however, I wouldn't expect it to be using that much CPU - and certainly not for more than very short periods of time. On my systems it'll occasionally jump up to around 40% usage, but drops back again fairly quickly.
It's worth taking a hard look at what apps you may have running in the background - not only stuff in login items, but software such as anti-virus applications that may have installed kernel extensions or background processes via /Library/StartupItems