DVDBurning - 3:04 pm on Jun 30, 2004 (gmt 0)

johnlim,
Do your sites link to each other? Are they on the same IP address, or in the same C class (IP addresses are identical for the first three octets AAA.BBB.CCC.****)? Chances are, you've been caught by the new "network of sites" filter. If so, you're not alone. While it used to be just fine to cross-link between sites owned by the same person or organization, as long as you were not duplicating content, there seems to be a filter in place that penalizes crosslinking. This may not apply in all cases, such as when one of the sites has a high PageRank (6 or greater seems to help avoid some filters).

If this is not the case... check the Google Terms of Service for other possible reasons that you could have received a penalty.