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Double-check that the "Auto-Rotate Screen" is still checked under Settings/Display. From a settings standpoint, that's the only thing I can think of. From that same screen, you can also try the horizontal calibration, in the (probably unlikely) event it's not set correctly.
Beyond that, it could be a hardware or software defect on your device.
\I believe Auto-Rotate Screen option would eliminate landscape as an option, keeping the phone in portrait.
Horizontal calibration is one pretty good option. If that doesn't work, you can try a reboot and/or battery pull. Otherwise you could try calibrating your compass...

Any other input? How would I go about doing a soft reset on the phone?