There is a difference in the order you select the lines to get the trim command to work.
Suppose you have 5 lines that are all intersected by another line (call this line Joe). To trim all of the five lines at Joe you need to have the 5 lines selected. Then hit cmd T and click on Joe. Voila, the five lines are split and Joe remains intact.
If however you want to trim one line at Joe, you need to select the objects in the opposite order. So now select Joe first. Press cmd T and click on the one line to split. This will still leave Joe intact and split the unselected line. This FEATURE has been this way since MiniCadd.
I think this might be why you are having trouble with the trim command. I find that this works whether Joe intersects the other line or only butts up to it.
[This message has been edited by Hurtz (edited 02-17-2000).]