Lower levels (for girls) are still scored out of 10 in the US.
Here's how the scoring system works:
You get two scores. The first is your execution score. You start with 10, and any execution deductions (ie form breaks, falls, etc) are taken from this score. The second is your difficulty score. There is no solid limit on this score. The harder your routine, the higher this score.
That article makes this easier to understand, but with one flaw that bugs me every time I see it-the last line. A 17 is not comparable to the perfect 10, because a gymnast can score a 'perfect' 17 while having errors in her performance. Take Nastia Liukin for example. Her difficulty on bars is 7.7, so the actual perfect score for that routine would be a 17.7, while Shawn Johnson's perfect 10 on vault would actually be well under a 17.
Like I said before, the number to shoot for will vary as difficulty values get raised, barriers are broken, and limits are extended. Right now I believe Beth Tweddle of Great Britain has the highest start value on any apparatus-a 17.8 on bars, if she executes her upgrades cleanly. But I could be wrong. I have a feeling it won't be soon before we start seeing 8.0 difficulty values, which would make 18 the new 10. Especially if Nastia's hints at the 2012 Olympics turn out to be a reality. And if He Kexin keeps going too.