clamassassin-1.2.4 has been released to fix a compatibility problem with clamav-0.90. changelog: http://drivel.com/clamassassin/clamassassin-1.2.4/CHANGELOG
Renaming clamassassin-1.2.3.ebuild to clamassassin-1.2.4.ebuild seems to work for me (with clamdscan here). 1.2.4 should probably depends on >=clamav-0.90 (see changelog).
In CVS, with updated clamav dependency. Thanks!