I've been seeing this occasionally for years. As far as I can tell, it's always been because my view was nearly edge-on to the plane the tab key would cycle to next. Rotating the view will free this up. BTW, 2D sketches will prevent the creation of sketch entities under similar circumstances.
I hope it's that simple for you in '09.
As far as I know, the only time this will happen is if you are looking straight down one of the axes. You will then not be able to switch to either of the planes that run through that axis. Rotate the model ~30° in any direction and it should become available.
In my case, it happens on isometric type views. However, going to an orthographic view will act as a work around.
Thanks for the comments.
I am new to SolidWorks and to Modeling in general, but I have been working though the Solid works tutorials as well as modeling some components for Uni, but am stuck on the tutorial 'Using a 3D Sketch to Create an Oven Rack Frame', requiring me to 'TAB' to the next plane. I have tried rotating the view, isometric views etc but I have been unable to change to another plane.
I have tried changing the plane with the line tool on or off, If the view is around 60^ from the original sketch plane (or more) I am unable to start a line at all.
It is starting to become very frustrating as I still have a lot more to learn.
It sounds like you are trying to sketch in 3D in a 2D sketch.
Make sure you start the sketch by pressing the 3D sketch button, which is different than the typical 2D sketch button. Then when you draw lines, you will see the small red axes after you click on a point. Pressing tab will rotate these axes so you can then sketch in a different "plane".
If you are still haveing trouble, can you take a screenshot of what you are seeing? Please include the entire graphics area, then save as a .png (if you can, else .gif or .bmp will work) and upload here.