For those that don't know a lot on the subject - you might want to realize that there are two different realms being discussed in this thread.
The OP asked about VOIP as a backbone for his phone system - which I personally believe to be a terrible idea at this time, unless implemented by a non-consumer corporation, with a reliability guarantee.... and maybe not even then.
A lot of you are talking about VOIP for internal communication, though. That's an entirely different can of worms - which may or may not make sense for this company. Logic would dictate that they already have a PBX in place... assuming it is serving their needs, I see no reason that they would benefit from using VOIP telecommunication equipment internally, except for possibly the reduced infrastructure costs, if they were to place the voice communication traffic on the same switched network as their data traffic - then there is the possible for a nice amount of savings, especially if expansion will be key in the future. That saving would recoup itself in the long term, though - it certainly would not save anything immediately. Most companies that are using VOIP are using this - they implement VOIP internally, while they still use traditional telco services on the other side of the demarcation point - such as a T1, or multiples of single analog circuits, etc.
At my last job, I managed a 3Com NBX 100 system (VOIP PBX) at one location, and a Nortel Norstar (Traditional Switched Circuit PBX) at one location. The location that had implemented the VOIP system was FAR easier to manage. The voice traffic was running on the already existant data network - no infrastructure additions were needed when we moved from a legacy PBX to the NBX - we happened to have a network drop (read: computer) at each location that was to have a phone. As long as we got a network drop there, the phone system could talk to it. I had no issues with reliability of the system internally (except for administrator error when initially implementing it, but that is no fault of the product) - it has performed extremely reliably. We had over 100 internal extensions, not including 3 or 4 fax machines and other analog devices (which we also attached to the network, through an analog terminal adapter).
Externally, we had a T1 for most of our voice traffic (utilization usually around 50% during business hours, but during emergencies, becoming fully utilized), as well as other individual analog circuits implemented both as a failsafe / emergency communication circuit, and for devices that had pre-existing circuits (that we simply attached to the NBX). Even when at peak usage, the system had no performance issues (I've heard there were performance issues with older VOIP PBX's when utilization was high). If internal voice communication over IP is in your future, I suggest you contact both your 3Com Rep and your Cisco Rep for a demo of each of their VOIP products - I have had excellent experience with both. The Nortel Norstar system, mentioned earlier, was a PITA to manage, just because of infrastructure issues in a pre-existing environment, and it is just somewhat more of a PITA to manage (the NBX was managed via a web interface to the system; the Norstar was managed by using key sequences typed in on the operator's telephone). The Norstar was my preferred PBX, before internal VOIP started becoming more mainstream and stable. I now greatly prefer the internal VOIP solutions by either 3Com or Cisco.
Since I'm tired, this post may have been as clear as mud. I'll clarify if needed.
<edit>
I should point out that the NBX is able to communicate with other NBX's (or by itself, to other phones) via WAN data links - the building that has the Nortel system, as well as one other, is scheduled to roll over to the 3Com NBX as soon as possible - the equipment is ready to go, just need to roll out the phones and train the users on the new phones... it works excellently as a WAN VOIP communication system. I know that Cisco solutions (and I would imagine all other VOIP PBX's) have the same ability.