Of the many phones/MIDs/ARM gadgets I evaluated before I got my Vogue, the only ones I saw that had even remotely open OpenGL drivers were based on TI's OMAP3 SoC or had a PowerVR SGX GPU. Unfortunately, none of the OMAP3/PowerVR devices I saw were cheap (OpenPandora, AI Touchbook, BeagleBoard, Nokia N900, etc.) enough for me. That, and I saw what happened with the TouchBook's OpenGL ES library, which apparently wasn't allowed to be distributed outside of TI's SDK - but I haven't been following that. I also saw that the Samsung S3C6410, used in the cheap made-in-China SmartQ5 and Q7 MIDs, has open enough specs for writing a driver, but no one has stepped up to write one yet. Aside from OpenGL, though, an OMAP3/4 based phone would be perfectly open... except there aren't many consumer OMAP3 phones
I really wish reverse-engineering or converting the Qualcomm/ATI libhgl.so for "real" Linux wasn't next to impossible/illegal - if doing it was easy, you'd have an OpenGL ES library for Debian on the Dream by now. I would reverse engineer it if I had the resources, unfortunately I'm unsure how legal it would be to do that.
EDIT: as far as phones (as opposed to the non-phones I was talking about), the most open right now seems to be Qualcomm - not counting Marvell PXA or other feature-poor (opposite of feature-rich :P) SoCs - as contradictory as that may seem. If you haven't guessed by now, I'm basing everything on OpenGL drivers, since as far as other hardware goes, I don't have much expertise. Also, I haven't looked hard enough to find any Freescale- or other ARM SoC-based phones, and I don't know of any Android phones (shipped with android, not ported by third-party developers) that DON'T use Qualcomm chips. For the moment, it seems you must pay a premium for openness.