An option that needs to be explored is using VMs such as either MS Virtual Server or VMWare.  This would go a long way towards ameliorating the concerns in the article with regards to patches required for one app conflicting with another.  Other miscellaney software issues that can crop up only affect one VM at a time.  VMWare long ago passed the point where it just runs along smoothly without problems in itself so as long as the host server is of sufficient enterprise quality and isn't pulling other duty.  The host system's downtime is a very rare occurance.  I've used multiple SQL servers on VMs within a well equipped, quality host server and it works quite well as long as the time is taken to properly allocate the resources for the individual VMs via the host VM management.  Overselling the CPUs for example (such as 8 VMs each allowed to use 2 CPUs each in an 8 way) functions well under light loads but leads to performance issues when all are used heavily enough that the VMs fight for time on the real CPUs.  But the same server with only 4 VMs allowed to use 2 CPUs each could run at reasonably high loads.  The same applies to disks, RAM, etc, etc, but you get the idea.

The problem with this idea is that it eliminates the cost savings in software licensing for the OS and SQL Server as each VM needs its own license.  But in hardware the savings are still considerable.  Also, such licenses as backup software are still able to be consolidated.  You only need one backup license to be run on the host OS.  Its also easier to manage only one host physical server sitting in the rack even though there is still the same level of software management of the individual servers within the VMs.

Anyway, just my 2 cents on the consolidation issue.  Your mileage may vary.