What OS are you using? I remember that happening to me in .75.. I haven't payed attn using .99. I know one of the leaks I installed it would stay in portrait mode when I layed it down.. I can't remember which one though. it could also be the accelerometer and not the OS.. just my .02..
since u have a 9530 from Verizon 9 outta 10 says ur using OS.75 ( as i am as well )..its a known bug. hopefully when OS .99 becomes stable, that'll be fixed.