this is one of those things that when it occurs to you it seems incredibly obvious and simple, but obvious things aren’t always so obvious for some reason so i thought i’d share…ever needed a simple way to change the interpolation type of your camera in the middle of an anim? no problemo,…set camera ipo to bezier, and parent the camera to an empty with ipo type constant. doink! ( bonks self on head with shoe )
Need few minutes to understand, and then… Oh yessss, good trick 
Tnx Modron 
That’s absolutely fabolous, Modron. Of course, it’s not possible to merely change the interpolation type of an IPO between points…
yeah i have been fooling with it and the results are quite cool,…also, you can add an additional empty for linear. [!]
Modron, hint: When reading intrr’s posts, make sure your sarcasm detector is correctly tuned 
You can press H, Shift H, or V to change the type of each handle in any curve (3d, or in Ipo editor). You can mix any of these handles inside the same curve, no need for screwing around with empties.
For some absurd reason, this seems to be missing from the Ipo menus in 2.33, though it was there in 2.31 (and perhaps 2.32, don’t have that version on this computer). /me hunts down the culprit …
actually, intrr is wrong, you can’t change the interpolation type on the same curve. What you can do is have linear section on a bezier interpolated curve, but you can’t mix biezer interpolation with constant interpolation (like Modron suggested).
Martin
i wasn’t suggesting that you could, but that it could be simulated by using an empty for the constant one. anyway, i didn’t even know that you could have bezier and linear at the same time, though i guess that could also be accomplished by scaling the handles.
in order to dispel any further confusion here is a .blend with an example.
http://home.att.net/~moton-toybox/constantbezierIPO.zip