As I've written previously, there are perhaps more differences than meet the eye between a photo site like flickr and one like iStockPhoto. The former is far more about community, the latter about commerce. Conflating the two as both representing the same flavor of "crowdsourcing" therefore seems like too much of an oversimplification to me.
However, the incredible popularity of flickr--and the great quality of many of the photos contained therein (including by professionals)--probably makes it inevitable that Yahoo! (who owns flickr) would look to ways to make the site more pro-friendly and pro-enabling. After all, there's almost certainly far more money to be made off those using a service for business than as a hobby.
And indeed, Stephen Shankland of Cnet news.com yesterday pointed me to some speculation about flickr getting into the stock photo business over at Dan Heller's Photography Business Blog. Stephen's thoughts are here. I pretty much agree with his take. iStockPhoto is constituted to make it very easy for buyers by manually vetting every photo for quality, model release forms, lack of trademark violations, etc. And it would be a mistake, I think, to minimize the usefulness of what's effectively a service level guarantee that iStockPhoto (and the other microstock sites) provide. A lot of "stock" may indeed "often seem so oversaturated and artificial and sometimes creepy" (to quote Stephen) and remind us eerily of the default Windows desktop. But when you're a graphics director putting together a corporate presentation, you tend to be far more concerned with issues like technical quality and legal safety than you are about ars gratia artis.
Thus, I'm not convinced that there's a great fit between flickr and the sort of commercial stock that tends to dominate iStockPhoto.
That said, I do think that there are opportunities for flickr to get more pro-friendly. One need only look to some of the features available on a site such as SmugMug, for example. It offers custom watermarks and you can choose which resolutions of your photos to make public (unlike flickr where it's all or nothing). One could also imagine more expansive file size limits and the ability to upload RAW format images for archiving and backup purposes. You'd also need to add some way of indicating that model releases were available and probably some sort of reputation mechanism a la eBay's feedback. This is all very do-able.
More difficult would be figuring out when and how to involve humans on the flickr side. As a non-commerce community site, flickr needs very little in the way of staff to resolve conflicts between members and over transactions today. That would rise exponentially were it to more explicitly weave the filthy lucre into its model (and hopefully without causing too many problems with the successful community that it's built to-date).
For the reasons I discussed earlier, I don't see a more commercially-oriented flickr as a replacement for iStockPhoto. It may evolve to better enable the sorts of services and features that would benefit pros, but I think it likely to remain a somewhat messy Web 2.0 bazaar. In other words, a site with lots of great bargains and art for the adventurous but caveat emptor.