Ideal humidity for a server room?

I've always heard 40%, though I can't back that up. I will say though that you need some humidity to reduce static electricity build up.

EDIT:

Ah, I found my documentation, good old Sun Microsystems Part No. 805-5863-13, "Sun Microsystems Data Center Site Planning Guide: Data Centers’ Best Practices"

Temperature and relative humidity conditions should be maintained at levels that allow for the greatest operational buffer in case of environmental support equipment down-time. The goal levels for the computer room should be determined in a manner that will achieve the greatest operational buffer and the least possibility of negative influence. The specific hardware design, room configuration, environmental support equipment design and other influencing factors should be taken into consideration when determining the specific relative humidity control appropriate for a particular room. Psychrometrics can affect hardware through thermal influences, Electrostatic Discharge (ESD), and increases in environmental corrosivity.

And:

Under most circumstances, air conditioners should be set at 72º F (22º C) with a sensitivity range of +/- 2º F (+/-1º C). Humidifiers, in most cases, should be set at 48% RH with a sensitivity range of +/- 3% RH. The set-points of the air conditioners should always be chosen in an effort to maintain the optimal recommended temperature and relative humidity levels for the room environment. These set points should maintain appropriate conditions, while allowing wide enough sensitivity ranges to help avoid frequent cycling of the units. While these tight ranges would be difficult to maintain in a loosely controlled office environment, they should be easily attained in a controlled data center.

Numerous factors, such as heat-load and vapor barrier integrity, will influence the actual set-points. If the room lacks adequate vapor barrier protection, for instance, it may be necessary to adjust humidifier set points to accommodate seasonal influences. Ideally, all inappropriate influences on the data center environment will be eliminated, but in the event that they are not, minor adjustments, made by trained personnel, can help alleviate their effects on the environment.

And on Electrostatic Discharge:

The maintenance of appropriate relative humidity levels is probably the most universal and easiest means of addressing ESD concerns. Appropriate moisture levels will help ease the dissipation of charges, lessening the likelihood of catastrophic failures. The following chart illustrates the effect moisture levels can have on electrostatic charge generation. Note – Source Simco, A Basic Guide to an ESD Control Program for Electronics Manufacturers

TABLE 6-3 Electrostatic Voltage At Workstations
                            Static Voltage 
Means Of Static Generation  Relative Humidity 10-20% Relative Humidity 65-90%
Walking Across Carpet       35,000                   1,500
Walking over vinyl floor    12,000                   250
Worker at bench             6,000                    100
Vinyl envelopes for work instructions 
                            7,000                    600
Common polly bag picked up from bench
                            20,000                   1,200
Work chair padded with urethane foam 
                            18,000                   1,500

Whilst you're right about a small degree of humidity helping a little I'd still try to avoid any more than can be cheaply removed. Certainly I wouldn't plan a data center based on the assumption that my heat-scrubbing capacity was reliant on that humidity effect. Design for the lowest humidity and get some of those low/high/average/current alerter things in two corners of the room for environment tracking and look at them every few days to see if things are changing over time.

The American Society of Heating, Refrigeration, and Air conditioning Engineers (ASHRAE) released a whitepaper in 2011. It says:

• Recommended: 5.5ºC DP to 60% RH and 15ºC DP
• Allowable: 20% to 80% RH