I think evidence points to this being true. For instance, in Australia - a technically advanced market where the iPhone is available on every major wireless carrier - the carrier exclusivity doesn't warp market share in Androids favour:

http://www.theaustralian.com.au/australian-it/surging-iphone...

As of May 2010, Android is at 2.1% smartphone market share, the iPhone is at 40.3% smartphone market share.

In the US, the iPhone's AT&T exclusivity gives Android protection from true competition. When the customer can select a phone on its own merits and not due to network availability I'm sure that the market share picture may be different.