Well, thought I'd talk about this a little bit since this is something that isn't very clear cut. While temperature and humidity levels can vary among all server rooms depending on the number of servers there are, other hardware, type of room, size of room, location of room, etc. Most people aren't sure what the ideal temperature and humidity levels should be to properly cool their equipment.
I can say that from my experience it varies a lot, but there are general guidelines to follow. Most of the IT community (from talking with individuals in the field and through sources on the Internet) agree that the server room should not exceed roughly around 28 degrees Celsius in general. I think that's way too high in my opinion, 28 degrees in a room is pretty warm!
I've had the server room hit 26-27 degrees before and to me it was really warm. APC wrote a white paper to give their general guidelines (http://www.apcmedia.com/salestools/VAVR-5UDSLG_R2_EN.pdf). They say around 22-24 degrees Celsius is adequate with about 35%-50% humidity. Sun Microsystems in a articles (which I cannot find anymore) suggested around 21-23 degree Celsius for temperature.
From what I've read and heard so far and my own opinion on the matter, "normal" server rooms should have temperature of about 19-23 degrees Celsius and about 30%-50% humidity.
Minimum temperature I would say don't go below 16 degrees Celsius and for maximum temperature don't go above 25 degrees Celsius.
Server rooms should always be "cool" and "dry", but not "too cold" or "too dry". Anyway, that's my thoughts on that.