I cannot comment on this specifically for the N900, as I don't have a N900 and have not seen even something as simple as a block diagram of the hardware or a list of the major parts that are in it. As soon as somebody provides that or high-res PCB photographs, I'm happy to verify if that is the case. From my past experience with all the phones that I've spent time reverse engineering, it has not been true!
2) modem baseband firmware / interface
as stated, I can live with the proprietary baseband modem software (just like I have to live with proprietary GPS and bluetooth firmware), as long as the interface is documented.
Yes, with PhoNet in the kernel there is some code. What I've heard from a number of people who have worked with nokia firmware based modems, they tell me that PhoNet in the kernel is incomplete.
Also, I have never seen any documentation on that protocol, neither the lower layers (PhoNet), nor the higher layers that are being implemented in ophono now.
3) battery charging logic
that sounds like a really strange bit to keep proprietary. I doubt there can be many trade secrets in there that need to be hidden. So it is completely unacceptable to have something as simple as that proprietary. However, if the hardware is reasonably well documented (i.e. which gpio is attached to which external piece of hardware, how does the battery charger / coulomb counter work, ...) then somebody can write a replacement.
So far, I yet have to see any of that for the N900.
I think there is a common misunderstanding of people who mainly work in application space about how many hardware details you really need to be able to actually write your own free software drivers without reverse engineering. No offence, this is not about you personally but about many of the statements I see in the community.
4) Lack of open source SGX driver
Unless the various companies using SGX cores don't _demand_ a driver solution that is in line with the regular linux graphics architecture and that is at least open source 'friendly', Imgtec will certainly not change.
All I'm seing is that Intel has been hit hard by the Poulsboro debacle, as well as everyone who builds products on that chipset. I know of other semiconductor makers who are struggling very hard with their customers requests vs. what Imgtec can provide.
I also know that the TI OMAP contains the Imgtec SGX core based on customer request... I presume that customer was Nokia in the first place.
5) serial console / jtag
Another thing that is key for actually doing efficient/reasonable system-level development on any hardware: serial console and jtag pinout. This information has never been officially released by Nokia,
not even for the previous n770/n800/n810. I know somee people have discovered the serial port, but where is JTAG? Doing kernel-level or bootloader work without JTAG is an order of magnitude harder. If Nokia cared about enabling a community to work on that level of their product, they should disclose that information.
On the general openness of the N900:
_If_ it is really true what you are writing, i.e. the documentation is all there and there are no other closed bits apart from bootloader, battery charging, SGX driver and baseband firmware: Yes, it is more open than most what we've seen before in that market. But given the hype that existed about every new device, including the previous Nokia nXXX products, I am extremely careful with my enthusiasm. So far, all of them have not really been very exciting from an openness point of view.