Bluetooth vs Bluetooth
Bluetooth should handle this reasonably well. The bluetooth radio layer actually frequency hops over 79 different frequencies spread 1MHz apart. Even with many people using them near each other, they shouldn't notice any degradation, and anything they do notice will be similar to the occasional packet drop over cellphones that doesn't impede communication at all. They hop at close to the same rate, and back off when they sense interference, which means that multiple bluetooth devices near each other will seamlessly synchronize so they are on different frequencies all the time with few collisions.
Bluetooth vs WiFi
Read this HP document to learn the details regarding WiFi and Bluetooth coexistence. To summarize:
Bluetooth interference from WiFi
In short, a single Wifi channel will interfere with about 22 of bluetooth's 79 hopping channels. In voice communications the packets will simply be dropped, but the modulation schemes are different, so this should only occur where the access point has a very strong signal.
WiFi intereference from Bluetooth
The access point, in turn, will slow down communications as it sees the bluetooth interference. Due to robust TCP/IP protocols most of your users won't notice anything other than slower access, however some realtime traffic (VOIP, video, etc) may suffer slightly greater packet loss since they won't be retried if lost.
Bluetooth automatically adjusts radio power according to its needs, this means that if the user is very close (within a few feet) of their phone, then the bluetooth signal will be very low power, and therefore interfere with very little. If they walk across the office, then the radio pumps out more power and interferes with many other radios nearby.
Testing will show whether this is a problem in your situation.
Alternative solution: 802.11 A 5.8GHz access point
You might consider using 802.11 A for your wireless network, which won't interfere with the bluetooth at all, being in the 5.8GHz band. A/B/G/N all in one wifi cards are common and cheap now, so there's little extra cost in going this route.
Yes, you will experience measurable interference. However it may not noticeably impact your operation depending on many, many factors, such as the network utilization, bluetooth utilization, radio strength, cubicle material, building composition and materials, nearby microwaves, etc, etc, etc.
In many cases this situation would work out fine, especially if you have few low bandwidth users on the wireless network, and only a few people chatting on the bluetooth at once.
In some cases regardless of other parameters you will find the situation unbearable - such as with the latest 802.11 N radios, which bond multiple channels and therefore saturate the 2.4GHz spectrum if users are using all 450mbps (or 600mbps if those radios are out yet...) and you've got 30+ people on bluetooth inside a single conference room with the AP in the middle.