How to invoke the "New Changelist" dialog?

Just create a new instance of com.intellij.openapi.vcs.changes.ui.NewChangelistDialog and show it.
I don't think there's an API for setting values on it though.

com.intellij.openapi.vcs.changes.ui does not seem to exist.

It's in idea.jar.
You'll have to add it to your IDEA JDK.

is it legal?

Yes, it's totally legal. The only additional constraint is that you can't distribute idea.jar in a product of yours.
But if your just implementing a plugin to run in IDEA you have no problems, you can use all the classes in idea.jar.

Hello Janusz,

I would like to invoke the "New Changelist" dialog. Is this possible?
If yes, is it possible to pre-fill it with some text in the title and
comment fields?

The dialog is called NewChangelistDialog. There's no API for prefilling its
title/comment.

You can also implement the dialog by yourself and use EditChangelistPanel
inside it, which will let you set the title/comment.

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Dmitry Jemerov
Development Lead
JetBrains, Inc.
http://www.jetbrains.com/
"Develop with Pleasure!"