This is very specific to 2D data. I bet the story is a lot different for high-dimensional data. The challenges you encounter with this sort of clumping in 2D is unlikely to occur in high-dimensional data due to the "curse" of dimensionality. Clustering in high dimensions has its own quirks and gotchas too, but they're quite distinct from the gotchas of low-dimensionality.

Here's more from an actual expert: http://research.microsoft.com/en-US/people/kannan/book-chapt...