So.. What can I say? Copy & Paste owned my hiney.
In my previous post, I was bewildered by a behavior I've been seeing where a Dialog box opened from another dialog box would cause the first dialog box to close.
Well, this is partly because of Copy & Paste and partly because I am a moron. When designing forms, I usually copy & paste buttons around since it's the easiest way of having the same exact size for all of them (what can I say, I am lazy). This has the obvious effect of causing all the properties to be copied over. Including the DialogResult property (duh). So I ended up with a bunch of buttons that would close the dialog. So, as (almost) always, .NET is doing exactly what I am asking it to do.
This also explains the fact that when I searched for this behavior, some people were reporting it and others were unable to repro it. It also explains why it only happens "sometimes". Lesson learnt.