I think you are putting the cart before the horse here. I would recommend choosing the platform first and then worry about the operating system afterwards. Be aware even with the same kernel and user-space versions from the same vendors, there are significant differences between x86 and ARM systems - particularly in terms of power management and drivers.
Kernel version 2.6.32 is also pretty old - in fact ancient in the fast moving world of ARM SoC. New functionality tends not to be backported.
In terms of best supported ARM SoCs at least the following are factors:
- A particular kernel tree (almost never main-line) supports all of the functionality of the ARM SoC
- [Paid for] Commercial or vendor support is available for the SoC
- Wide choice of COTS systems available using SoC.
- Full system support in OS distributions
- Supported by developer/enthusiast community
Taking Pandaboard as a case study since it's been mentioned by several other posters:
- Most device functionality is supported in recent mainline distros
- Wide community support - although very little of the documentation is authoritative or maintained and answers can be hard to find. You'll be on your own if you get stuck.
- You can't buy Pandaboard for production use, it's sold as a evaluation board with little vendor support.
- You can't buy Pandaboard in volume.
- TI don't provide support for OMAP4 unless you're a tier 1 OEM, although most of the datasheet (7k pages, 44MiB) is public. Other parts under NDA.
It's fairly clear from your description that this doesn't fit your needs. I would therefore suggest looking at COTS systems.
I've had good experience using products from PhyTec, who make a number of different ARM SoC-based systems (in some cases, on interchangeable modules). They ship their systems qualified with Pengutronix's Linux Distro, although there's nothing to stop you using their kernel and somebody else's user-space (I used Angstrom). PhyTec's kernel patches are contributed to the linux-omap tree. [5]
There are plenty of other vendors of similar COTS systems.
Needless to say, these COTS board are not directly supported by mainline distros, although in practice are fairly close.
[5]: Linux mainline is worth avoiding for ARM SoC.
