Unfortunately, WordPress.com blogs don't allow 3rd party plugins outside of the widgets offered on the service itself. However, Automattic (the company that created WordPress) recently acquired Intense Debate, which is a competitor to Disqus. That's good and bad news; good that WP will eventually get a Disqus-style centralized comment architecture, but bad that support for Disqus specifically on WordPress.com will probably not happen now.
All that said, if you do decide to go with a self-hosted WordPress blog, it is unlikely that the open plugin architecture it has will change dramatically. So you can safely depend on Disqus being there and supported as WordPress grows and develops its platform.
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All that said, if you do decide to go with a self-hosted WordPress blog, it is unlikely that the open plugin architecture it has will change dramatically. So you can safely depend on Disqus being there and supported as WordPress grows and develops its platform.