I don’t think this is going to be much of a threat to MySQL or PostgreSQL users. What Oracle should have done is offer the depreciated 8i Enterprise Edition instead of dumbing down 10g. Basically this is a ploy to get companies hooked on Oracle and then require companies to upgrade to 10g Enterprise Edition when their applications require it. Don’t get too excited about installing ArcSDE with it as the limitations of Express Edition should make it practically unusable. Standard Edition 10g is still a better choice and at about $149 per user, not that expensive.
I still think ESRI should be looking at a version of ArcSDE that works with PostgreSQL but maybe that time is soon.