The macro sites will want more mp's but I am not so sure about the micro sites.  I have read several times that a very high percentage of our photos are used only on websites.  Most of my photos that I have found in use would not have to be more than about 4mp. 

I once had a photo from my 2mp camera printed on the front cover of the Observer travel supplement.

Perhaps 10mp will be a minimum one day but I think it is the quality of the pixels that counts more than the number.

After upgrading to a 16.7mp camera last Spring, I can honestly tell you that you are at a competitive advantage with making such a large investment and using a camera that shoots at larger sizes.

1) Your image quality is MUCH better when you submit to traditional agencies;
2) You have the ability to sell at larger sizes (some will argue this isn't that big of a deal - I disagree)
3) You have MUCH more flexibility with what to do with an image.

In the case of the micros, #3 is the most important part.  I'll give an example...I recently shot an image of a fly fisherman on a pond.  I tried uploading an un-cropped version at full size to various agencies.  I got virtually the same rejection message across many of my top selling agencies - "lack of composition".  Though the image was accepted at other agencies.

I took two minutes, cropped the image down to 6.2mp and re-submitted it.  Almost every agency accepted the cropped image (Shutterstock didn't like the lighting the second time around).

I'm still selling at a decent size with the cropped image.  Honestly, I don't understand why we can't leave the cropping to the designers and give them the choice but hey - what the agencies want, the agencies get.

My next camera will be at least 22mp  (the new 1Ds Mark III).

I agree that larger/better/higher MP cameras offer the photographer better choices when they need to crop, etc.  But I still think that there are some incredible images out there from smaller res cameras and I don't see a real reason why the microstock agencies are slowly bumping up the minimum requirements.  Remember, was it a year or two ago, istock upped the size to be able to sell at "large" to 5MP.  I'm going to guess that in less than a year they'll raise that again, to 6MP in order to sell an image at large size, because all beginner's DSLRs now start at the 6MP mark.   The only credit I will give istock in terms of submissions though, is that they still DO accept 1600x1200 sized images.  You won't get more than a medium sale out of it, but that's ok.   I've taken images in the past with an old Olympus 2100uz that to this day sell on istock.  Granted, it was designed and built as a high-end camera during it's day, and not a cheap plastic 2MP point-and-shoot, but still, it goes to show that lower res cameras CAN produce "sellable" photos.

I don't fault the macro stock agencies for having pro standards in images and image size requirements.  I do however, wish that micro stock would loosen up just a bit considering as someone else said, so many of the images bought from these places are used for web or small print use.  I simply do not see a legit reason to have 4MP minimum requirements for microstock as some have... 2MP should be fine for a minimum if the image quality is up to standards.

On the other hand, there's a value for the agency to offer all image sizes a person *might* want, once they've found the perfect image in the search engine. Offering low-res-only for some images would mean disappointment for some customers.

I've had an idea in my head for a long time and I just know someone will steal it and go with it if I post it, but what .. I'll never have the know-how or funds to do it myself, dammit...

Someone should come out with a "ministock" site.  A site that allows images from cameras as low as 1.3 megapixels; point-and-shoot - even camera phones.  Images would be inspected and judged for admission mostly based on composition and subject matter, with less emphasis put on pixel peeping for tiny flaws.  Images would focus on stuff just for web use, and again, be in the 1.3-5MP ranges for the most part, although pics from any camera would be accepted.  Prices would be 50-cents for the tiny stuff and up to $2.50 for the 5MP stuff or higher, with photographers getting at least 50% commissions.    No higher cost than $2.50 (excluding special licencing prices) which would actually discourage  "pro" users (real or in their own minds) from saturating the site, as they have all the others to chose from to sell on.  Contributors could request Paypal payments at ANY time with no minimum accumulated commissions.  This would be a site for "everyday" people or more specifically, every camera users, to sell THEIR pics on, without pixel * treating them like digital scum because of their gear.

Sound good?  I think there's room for it.  I think people would actually buy from it for casual photo needs, too.   Ok, so if anyone wants to invest in that kind of thing and do it, go for it.  Just remember where the idea came from and be decent and cut me in on some of the profits so I can pay off my Visa Card, ok?  LOL!