There doesn't seem to be a single standard that all vendors follow. If one vendor fits 48/96 blades in a 42U rack, another fits 60. The dimensions of the blade enclosures themselves are different, meaning you can't have a mixed vendor environment in a blade solution.
Ordinary rack servers on the other hand don't have this problem. You're not restricted to purchase racks from a select few vendors. You can buy various configurations at various price points. This gives you the flexibility of choosing what you need and from whoever you want.
The other challenge in blades is to get the application support for the same. As the highest number of CPUs per blade is still 4, you can't have a large enterprise application requiring more than 4 CPUs to run on the same. This problem could soon be taken care of with multi-core CPUs coming into the picture.
But with blade prices coming down, they're becoming a more feasible solution even for medium sized organizations. One problem that will always remain is that of server density. While it's good to have more computing power in the same space, it also puts more weight per square foot of floor space.
Isn't there a way to carve virtual machines out of multiple blades? For instance, if I have 60 blades, can 30 be dedicated to one task and the other 30 to another?
...And the fact that the heat they generate could warm a smallish city.