Might I ask why? I don't remember anything from linear algebra that required you to rotate a matrix by 90 degrees. Unless you're doing "rotation matricies", which, in 2-D are given as:

[[cos $theta, -sin $theta]
 [sin $theta,  cos $theta]]
[download]

Then again, you may not be doing linear algebra at all, so I might be off my rocker.

thor

Update: had the signs of the sines switched. Fixed.