On the FAQ page of its website, even the AHA recognizes that if you only know Hands-Only CPR and you discover an adult already unconscious, you will need to do something. The organization recommends Hands-Only CPR in that case, but it ignores the issues of kids and drowning victims.
There's no substitute for proper CPR training, but not everybody has that. If the only thing you are willing or able to do is push on the chest until help arrives, then push on the chest.
If you do nothing, a victim of cardiac arrest will die -- regardless if he or she is a kid, an adult, or a drowning victim. If you do something, he or she might live. It's that simple.
The AHA is the leader in cardiac arrest treatment science, but there are other cardiac experts who think that Hands-Only CPR has more uses than what the AHA is recommending. In fact, I believe the AHA will upgrade its recommendations to focus even more on chest compressions and less on rescue breathing in the 2010 CPR Guidelines.
No matter what: take a CPR class. Proper training might mean the difference between life and death for someone you love or someone you just met.