I stumbled across an article "Redecorating Your Guestrooms May Spell Disaster for Wireless Signals"
by Bill Halpin, on the site of Core Communications, a company that builds WLANs for hotels and conventions. It includes a nice table of various kinds of stuff that is used for hotel rooms, along with their respective expected attenuation factors (in %) for WLAN signals. Silvered mirrors and other chrome or metal objects are worst. Some of this information might be useful for people other than hotel managers.
UPDATE: Core Communications was acquired and has become Swisscom Hospitality Services. To match the Swisscom look & feel, useful information such as the above article probably had to be removed.