First, let me say how jealous i am that you are already working in 2010. I'm still stuck in the 2005 and fading off into the distance in the industry's rear view mirror. Never the less,
When you do a plot preview, does it show the text in the right orientation? Also, what format, when printing to a file, do you print to? If you print to a file and open that file, does the text show to be in the correct orientation or is it already showing the text rotated 90 degrees? I'm trying to rule out whether it is a setting in your autocad or a possible driver issue in your printer.
One thing I've learned is the drivers in a printer can make a world of difference in how it interprets the plot file and what it looks like afterwards....though, i can't say i ran into an issue with the text rotating on a plotting.
hope this will help lead you in the right direction.