hmmm.
i don't think top mounted fans are good idea for most cases cause of the power supply right above most of your heat components (cpu, video card and sometimes northbridge). it seems like if you have top mounted fan in mid of case it would take airflow away from components that need it most. that's why the front to back makes sense to me.
i did have a case however, that had powersupply stand up down instead of accross like in most cases, this worked well, cause i could put my top mounted fan right above where the cpu was. gave me best temps. i no longer have that case and i haven't been able to get the same performance since.
here's a picture of the case that i had, (may buy again cause its soo cheap).
Link to Case
anyway, can you see how the power supply stands up towards the open side of the case. the cpu actually would be right on the other side of that powersupply. this leaves very good space above the cpu (on top of the case) for a blow hole.
let me know what you guys think of this.