Still not fixed in Unity 4.5.2. I've been able to reproduce it multiple times.

How disappointing. The Unity team makes adjustments that they personally believe must've fixed the issue, so instead of checking with the public to make sure it really is fixed, they simply mark it as such. How about you let everyone who actually uses and relies on the program to confirm that it's fixed, rather than coming to this half-baked conclusion on your own? This isn't just a harmless error that pops up once in a while and can simply be ignored, this is an error that CRASHES Unity and prevents people from making any progress, therefore causing some serious problems for time-sensitive projects. Just seems a little unprofessional to me.

The issue only appears to occur with individually placed trees, those created with a brush size of 1. Mass produced trees don't appear to cause a crash on deletion.

Yes this is not fixed but simply creating one instance of a tree at app start-up stops the crash

This issue is NOT fixed in the latest Unity. (April 20th, 2014)

This is not fixed!

Same in our projects. Not fixed.

This is not fixed - it still crashes when you delete the last tree on the terrain

We are doing an infinite procedural world and constantly create and destroy trees on the same terrain block as the player moves through the world. I trapped in Visual Studio nad shows the culling code to be the culprit. The details are in my comment here...
http://answers.unity3d.com/questions/362253/crash-r6025-pure-virtual-function-call-in-webplaye.html#answer-599056

This bug is killing us because our game is in Alpha on Steam and crashes frequently because of this.

100% reproducible.