27MHz wireless keyboards and mice are a huge support hassle due to short range and interference. They are also extremely insecure. I would not even allow them on the premises.
Bluetooth devices are designed not to interfere with each other. Because Bluetooth is low-power/low-range and it uses frequency-hopping spread-spectrum communications, you do not need to worry about cross-interference between headsets and keyboards/mice.
Of course, there are other potential sources of interference in the 2.4 GHz spectrum: cordless phones, microwave ovens, WiFi, etc. However, if your end-users are already using Bluetooth headsets without issues, you are very unlikely experience any new problems when you add Bluetooth keyboards and mice.
The only support calls that you are likely to get for Bluetooth keyboards and mice will pertain to dead batteries. ("My computer froze! The mouse won't even move!") These too can be prevented: at the same time as you purchase the wireless keyboards/mice, get a high-quality NimH battery charger (La Crosse is fantastic) and rechargeable AA batteries (must be ULSD, Sanyo Eneloops are best). Create a schedule so that each wireless mouse/keyboard user gets batteries replaced and recharged on a predictable, routine basis, before they go dead. It will save time and frustration in the long run.
Otherwise, wireless mice/keyboards have a real cost to your organization in terms of urgent support calls and lost end-user productivity every time the batteries "unexpectedly" run out.