I think this was possible, and I briefly checked the reference guide but I can't seem to find it.
In the Piano Roll view, how do you slice a note event at once?
For example, is it possible to turn a whole note into sixteen 1/16 notes without doing the Alt-clicking fifteen times?

Thanks

If you are doing one not. Use the Scissors Tool.
Have your Vertical Grid lines displayed to at least the 1/16 Grid Resolution.
You can now get an exact cut.
 
 

The Scissors Tool is now called "Split Tool" and is the same as Alt-clicking, isn't it?
I discovered that it's possible to split a clip repeatedly at a desired interval using the Split Clip dialog box, but I can't find a feature like this for splitting note events repeatedly in PRV.

Hmm, I wonder if that wasn't Sonar...  

If you just want a fast way of creating 16 16th notes through one bar (or more) I'd use the free hand tool though, hidden in the smart tool (or selectable via it's own icon). .. ..Here's the smart-tool-way of doing it:

• Select the smart tool
• Make sure the PRV-snap funcion is active and set to 16th
• Hold down alt and ctrl
• While holding down the left mouse button, draw a line where you want the notes to appear

But hey.. There's an even better way too (especially if you want to make a long straight line with notes)... :O
You can also, while holding down alt and ctrl, also hold down shift at the same time.
This will, instead of the free hand tool give you the line tool. You now don't have to be so steady on your hand while you draw.
Just click where you want your first note to be and the move the mouse (while holding down the left mouse button) to where you want the last note to be and then release the mouse button. 16th notes will no be drawn in a straight line from your frist the first selected position to the last one.

Thanks, I'll use the line tool then because what I want to do now is just create a few looooooong straight lines with 16th notes.
But I wonder what method users would use if they simply wanted to replace a number of long notes existing on different keys with 16th notes. I clearly remember splitting a note event repeatedly using a single command (maybe it was studio one).

I use CAL for this.  Specifically a "Divide Note" routine.
 
I don't remember if SONAR comes with a CAL script that does this.  I wrote Divide Note.cal for myself to help with making trills.