In Blender 2.49 or similar:-Select the angled plane and the object you want to cut
-Press 'w' to show Boolean Tools Window
-Select Union
-A new object that is a mix of the two objects should be created.
-You will need to clear the remains of the plane that are in the new object
In Blender 2.69 or similar:-Select the object you want to cut
-Go to the Modifiers tab
-Add a Boolean modifier
-Enter the angled plane object into the "Object:" field
-The "Operation:" should be set to Intersect
-Apply the Modifier
-Clean up any unwanted faces/vertices in the cut object
NOTE: In both versions of Blender, the direction of the plane's normal seems to effect the outcome of the operation/cut. Make sure the normal is facing toward the side of the object that you want to keep.
just curious, but is there any particular reason you don't use the newer versions?
I use an older version of Blender for creating Blockland mods. Partly because it's what I started out with, but also because I'm not sure the .dts exporter of the later Blender works with Blockland (or behaves the same as the old exporter),
And I'm to lazy to try it out to make sure.