Oracle XE is great because it is lightweight and "free to develop, deploy and distribute". Dietmar Aust has posted a thorough clarification on XE licensing, where representatives from Oracle have confirmed that XE is indeed free. (The post also includes some good security advice in the comments.) Oracle XE has its limitations, but I've previously posted some performance benchmarks for Oracle XE, which shows that it is more than good enough for many use cases.
Back to the topic of this post... during the Q&A part of the "Oracle Database 12c Launch Webcast" there were several questions about a 12c version of XE:
Will there be an Oracle 12c Express Edition? It is planned, but there are no dates yet.
Is there an Express edition of 12c available? Planned, but no dates yet.
Will 12c be available for Express Edition? If so, schedule? Planned but no dates yet.
So basically, we now know that there will be an Oracle 12c Express Edition coming out, but we have no idea when. Hopefully it won't take 5-6 years, like the wait between 10g and 11g... (and when it arrives it will probably be for 64-bit Windows, which was promised for XE 11g, but which never materialized).
But hey, it's free, and it's a great product, so it will be worth the wait! :-)