OK, I’ll do the honours! I don’t know if you’ve encountered the IPO window at all, so I’ll try to talk you through everything.
First of all, make sure your cube is selected.
Make sure you are at frame 1 (or whatever frame you would like the rotation to start).
Press I while in the 3D view - a prompt will come up asking you to Insert Key… Click Rot - this will set the cube’s rotation, at this frame, to be as it currently is.
Move 10 frames forward.
Rotate the cube by, say, 90 degrees in all three axes of rotation. Of course, if you want a bit of randomness, rotate it in a different way.
Press I again, and once more click Rot when it asks you what to save keys for.
If you now use the cursors to move between frames, you should find that the cube rotates between the frames that you set keys for.
Now press Shift-F6. This is the IPO window. Along the bottom axis is time, while on the side axis is distance/rotation/other. You should see three bezier curves, with colours relating to the RotX, RotY and RotZ labels on the right hand side.
Select all three curves using Shift-RMB.
On the menu bar, select Curve->Interpolation Mode->Linear. Your curves should now be straight lines.
Then, on the menu bar, press Curve->Extend Mode->Extrapolation. Your curves should now extend to infinity!
You may find your cube moves too slowly for your liking. If this is the case, right-click an individual curve, press Tab, and then select an individual point on the curve. If you make the gradient greater, the cube will rotate more quickly.
Hopefully that helps - I dare say you know all about the IPO editor already and I’ve given you extreme overkill, but never mind!