I have no idea about Mer, but I'm finding that the N900 has very good support for its hardware in the mainline kernel. We're definitely standing on the shoulders of giants here
[1] In N900 HW adaptation there is 2 problematic components that we are NOT planning to open as such; the other is BME, battery management entity from Nokia, and the other is OpenGLES implementation from SGX. You can use the device without openGLES, if you don’t care about HW accelerated 3D features. Without the BME the device is practically useless. (Fully functional MeeGo images containing those closed components will also be provided, thanks for asking..)