Here you go, only one Linpack process for all cores/threads: LinX 0.5.5
Tested only on P4 and Atom, so please, i7 owners, if you're not yet tired beta-testing, try this one and check, whether all cores are loaded or not.

KURTZ and everybody who wants to link to LinX (sounds funny ), I think (almost sure) this link will remain the same for newer versions.

No WMI calls now, should detect everything correct on all OSes.
Quote from changelog (yes, there's even a changelog now! ):

LinX 0.5.5
- improved threading control. 2 Linpack processes are created no more to load all "cores" on HT processors
- added tray icon option. When enabled, LinX will be minimized to tray
- small interface fixes