UX Movement - Articles on Interface Design
uxmovement.com
uxmovement.com
In the Western world, where we read from left to right and top-down, we also expect our UIs to follow that order: first you type something and then you click on a button. Also I don't know if these visual fixations are really relevant here. Power users will use the enter key, so for them it's the best, anyway, if the button gets out of the way. Computer novices, on the other hand, will look away from the screen at the keyboard, while typing.
Even Google places the search button at the right of the input bar. I am sure they have A/B tested every possible variation and this turned out to work best. Last but not least it's simply what we are accustomed to.
A followup question: It is indeed best when the button gets out of the way. What assumptions are safe to make about the computer literacy of your visitors (i.e. they know to hit Enter to submit)?
I realize that this depends on the target demographic, but we should have a set of standards, I think. It influences everything from UI streamlining to what content goes below the fold (can we assume visitors know how or when to scroll?).