The developers are currently moving the default Python version from 2.6 to 2.7, which involves rebuilding (and sometimes changing) a lot of applications that are written in Python or depend on it (e.g. for plugins, scripting, etc.). I am running Natty on 1 machine, and currently part of this migration is done, but not all of it. The result is that the latest version of several packages (that are already converted) can not be installed together with the latest version of some other packages (that have not been converted yet). That also means that building an ISO fails, hence no new "daily" CDs.
(To be sure somebody from the team that is responsible for building the ISOs should confirm this, but I'm pretty sure this is the main reason.)
This may also happen in other cases where applications that are installed on de ISO depend on each other or on a common library, and have version mismatches.