This issue occurs in Netbeans 5.5.1, 6.0 and 6.1 on Ubuntu 7.1 using java-1.5.0-sun-1.5.0.13 and java-6-sun-1.6.0.03.
All are basic Netbeans versions (first selection on left of download table).  
The O/S is a pretty basic install .. the only added software was java, netbeans and vmplayer.

This issue is that when presented with a file chooser it is almost impossible to open a directory.

For example.  In the ide select services, database, add driver.  I attempt to locate the Jdbc driver stored in my common
Javalibs directory.  I am able to switch to it's root directory but when I attempt to traverse through the remaining
directories I must double click repeatedly before the directory finally opens. Sometime it seems to go into "rename"
mode, in which case I must temporarily click a different directory before retrying.

I have done some googling on the topic and the only suggestion was to add ~Xdefault and *.multiClickTime: 500.
This made an almost unnoticable improvement (50 clicks vs 60 or so).

I think this should be a priority fix as it is such as core part of ide and Ubuntu has become the dominate linux desktop
so others will be experiencing the same issue.

I've been using Netbeans since v4 and have slowly converted all my co-workers.
This is one excellent piece of work.
Thank You.