Herman,
I noticed that the holidays lists I imported all contain the text 'US' which does not occur in any of my normally posted events. Therefore, you could use the inclusion or exclusion of that text as a conditional.
There may be other inclusions or exclusions in fields from imported event lists that differ from regular events (such as a required field,
hint) that could form the basis for a completely reliable conditional, as DanO has alluded to.
Although not totally applicable to your situation, here is how I prevent holidays and unconfirmed events from appearing in my public rss feed template (the custom field event_type is a required field):
<% next if ($EVENT->{'details'}->{'event_type'} eq "") | | ($EVENT->{'details'}->{'event_type'} eq "Reserved"); %>
Good luck exploring!
[This message has been edited by musicvid (edited April 12, 2006).]