This thread is very timely for me. I'm expecting my first Dc in late July/early August. As soon as I knew the due date, I knew that myself and DP would have to work hard with LO to get them ready for school at an early age. Barring other factors, I believe it is 100% achievable to bring our child up to feel confident and willing to learn (though I know it may not be as easy as that!).
I know many July/August babies who have had varying success at school. One DSis was late July and struggled from the word go - despite the same level of attention and word-game playing that I received before I went to school. But there was an added twist, she was a 'difficult' baby, who, it is now suspected with hindsight, was probably lactose intolerant and so cried constantly for years. As a result she was small (though now much taller than average), lacked confidence and rejected school to an extent. She is now, however very successful.
My DSS is a July baby and struggles with some elements of his school life because of what appears to be dyspraxia and some hyper-mobility. He is also very naive and seems young for his age, but is growing up quickly.
In both of these cases, there are/were other factors at play that differentiate them from average ability summer-borns - and we will never know whether their academic slowness is down to these factors or their age, alhtough I know that in my sister's case, her age was seen almost as an excuse and she wasn't pushed as much as older kids.
Sometimes, ability is slowed by age maturity. Sometimes it is slowed by other factors (eg preemies, those with early health/development issues). Sometimes both can be counteracted by lots of hard work by the parents and professionals.
There will always be a bias, but the bias will unwind eventually - the biggest focus should be on finding ways to positively encourage all children to work as hard as they can, and praise them for that rather than their 'abilities'. It has been shown (read Nurture Shock by Po Bronson and Ashley Merryman) that telling a child 'well done, you're really smart' is more damaging to self-esteem than encouraging a child for the effort put in.
And surely, bottom line here, summing up all the posts, is finding a way to raise academically confident children who believe in their own ability to try harder and achieve, whatever their current or perceived level.