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massivegreatness.com
massivegreatness.com
Mobile is a clear choice for apps targeted at the consumer market.
However, there is still huge opportunities in the B2B and enterprise market. It depends on the problem you're solving, but in most cases the desktop is still the natural place to start if you're going after business users. Examples: Basecamp, Salesforce, Kissmetrics.
Smart phones and tablets are decent for consumption but their main limitation is screen real estate and efficient user input (which usually refers to typing). It really is a case by case basis (whats the product, who are your users, etc) and usually the answer is that you're going to want to be well represented across all typical client types.