Rolfo,
thanks for the quick answer. If it would work like you described it would be simply fantastic the perfect solution for my task.
But unfortunately it doesn't.
Depending on the checkboxes at the keywordfield (IPTC Stationary Pad) it either deletes all existing keywords and writes {keywords:lc} or it simply adds {keywords:lc} as a keyword or does nothing at all (if none checkbox is checked).
I'm currently using 4.5.3 beta 4 on Windows XP prof.
You'll have to use my second suggestion and use an unused field to temporarily hold the lowercased keywords. For instance, let's use the Edit Status field. In the Edit Status field, enter {keywords:lc} and uncheck all other Apply checkboxes. Then apply the IPTC Stationery Pad. Now the Edit Status field will have your lowercased keywords. Next uncheck the Edit Status Apply checkbox and enter {editstatus} in the Keywords field and apply the IPTC Stationery Pad. Now your lowercased keywords will be in the Keywords field. As a last step you may want to clear out the Edit Status field. To do this, uncheck the Keywords Apply checkbox, clear out the Edit Status text field and apply the IPTC Stationery Pad.
That's more steps than I would like, but it should work.
-Kirk