Just to make things abundantly clear:
Inverse Telecine (IVTC) is used to reverse the film-to-video procedure (called, uhh, telecining). For NTSC telecine, the 24fps film frames (e.g. A B C D x6) are converted to 29.97fps video frames (e.g. AA BB BC CD DD x6) using 3:2 (or 2:3) pulldown. You'll note that because both video fields come from the same film frame, AA, BB, and DD look 'perfect', while BC and CD will show combing because they combine fields from two different film frames. This 3:2 cadence is quite predictable, so it's possible to drop a B and D field to get AA BB CC DD, and from that a nice progressive A B C D -- the same thing that was input. PAL telecine does 2:2 pulldown, so it's much easier to make progressive -- there are no annoying hybrid frames to deal with.
Deinterlacing is used to make field-based material (e.g. just about anything shot with a video camera, or produced for TV) progressive. Video cameras record separate, unique fields at twice the nominal frame rate (i.e. 59.94fps for NTSC and 50fps for PAL), so displaying both fields of a given video frame at the same time will show distracting combing effects. Unlike IVTC, it is impossible to reverse one procedure to recover the original whole frame, because it never really existed in the first place. The two fields come from two different instances in time. The best you can do is to have your computer do a bunch of math and try to smooth out the combing -- sort of picking an intermediate time between the two fields, say. This necessarily destroys some of the original information, but usually the tradeoff is worth it. That is, the end result looks smoother and easier to watch on a progressive display or display format than would the original video.
Decombing is deinterlacing. However, in AviSynth, Donald Graft's (aka Neuron2) DeComb is probably the best package that does IVTC. This is because DeComb is highly adaptive, and can do both deinterlacing and IVTC. Telecide() and Decimate() together, with appropriate parameters, can correctly IVTC film material and deinterlace field-based material even when they appear in the same clip. Doesn't happen that often with movies on LD, but it can happen a lot on TV shows, or anime.
So, to sum up, for LD OT conversions, IVTC is what you want. Stay away from deinterlacing!