What metadata wouldn't you want to share between LR and C1?
IIRC, in the LR3 catalog settings, if you deselect "Write Develop Settings To XMP For JPG, TIFF, And PSD" and "Automatically Write Changes Into XMP" then Lightroom will manage metadata and developing steps only in its own catalog.
If C1 has similar settings, then you
should effectively be able to work on the same exact files with two totally different results for each application/catalog.
http://livedocs.adobe.com/en_US/Lightroom/1.0/help.html?content=WS2D181F5C-4...E37CF33B9.html http://www.scribd.com/doc/51914111/151/XMP-read-write-options