There is a growing trend to use watts per rack as a better indicator of actual equipment density than watts per sq. ft., since sq. ft. is calculated differently by different people -- some include just the rack footprints, some include aisle and service clearances, others include the entire raised floor space. This leads to apples and oranges comparisons. Here is a good
white paper from the Uptime Institute, which gives the details.
However, people still do use watts/sq. ft. to describe and compare data center designs. So, for comparison, we just completed a research data center at 400W/sq. ft. This site will house very dense 1U and blade servers. Our current retrofit project is taking a 25 yr old general purpose data center and upgrading it to 100W/sq. ft. We do not plan to have very dense racks overall and we'll treat hot spots with auxiliary cooling. These values are the result of dividing the UPS capacities by the raised floor space. No mechanical load is included.
From people I've talked with, data centers are being built at different capacities for different reasons. Colocation sites, Yahoo and Google, supercomputer sites, corporate data centers, etc. -- all will have different needs, budgets and design specs.
Hope that helps.
Ken