As commented in http://moodle.org/mod/forum/discuss.php?d=118852 it would be great to definitively drop support for enums in Moodle 2.0. It involves:

1) Transforming all (7) current enums to "normal" varchar fields (upgrade step in 1.9 => 2.0).
2) Drop enums/enumvalues from all the install.xml files (installation)
3) Drop enum support from XMLDB / tests / Dictionary Moodle DB functions completely (leaving only the option to drop them)
4) Document it properly (in 2.0 release Docs, in DB Layer 2.0 Docs and in contrib-related Docs.
5) In Moodle 2.1... drop the option of droppping enums (used in 3 under Moodle 2.0). Moodle 2.1 will require Moodle 2.0.

My +1 is here. If nothing arrives against that in the very next days... I'll do it. Ciao