I have to agree with others here, it depends where you live a lot.
At night or a quiet time, anything over 15dBa is clearly audible. Anything over ~22dBa can be annoying, depending on your preferences.
At regular daytime, most people don't mind anything not higher than ~28dBa. If you have a lot of ambient noise, anything below 30dBa would be masked.
Then there's the extremists.
Quiet freaks consider anything over 20dBa to be loud. Some even 15dBa, but it's really hard to get a system below 15dBa because of the HD. (Re: freak: no offense, just first word that came to my mind
actually I'm a quiet freak if I could afford it.)
Extreme air overclockers use fans that are ~50dBa, like the vantec tornados. I'm sure they mind it a bit, but to them 30dBa would probably be more them acceptable once they've gotten used to 50dBa.
General reference:
Stock HSFs are usually about 25-35dBa
Quiet HDs around 20-25dBa (such as seagates) and regular HDs ~30dBa.
Another thing about fan ratings. Often they differ from manufacturer to manufacturer. 28dBa from company #1 may be fairly louder than 30dBa from company #2. The ratings are generally relative, but they are often measured from difference distances and scenarios. Relative meaning: 50dBa from one company is almost certain to be louder than 30dBa from another company, in whatever case.