[gizmo686]

This isn't quite true.

は has two distinct usages. One is to mark the topic of the sentence. The other is to mark the subject of the sentence with a contrastive connotation.

[glandium]

It also has a use for emphasis. And a few others uses.

[strongai]

And what does 'contrastive connotation' mean?

[glandium]

See the examples on http://www.jgram.org/pages/viewOne.php?tagE=ha-2