LiMo, AFAIK, is a proprietary stack running on top on Linux. Android and Maemo are MUCH MUCH more open source than it.

Basically, all the phone companies are lazy, so they came together to make a open-source kernel and middleware to run their platform on. Like TiVo, Archos, and other companies that use Linux, the platform itself is locked down with proprietary interfaces and the like, so its harder to hack in that sense.