how many blade enclosures can you fit in a rack?

Considering the power and cooling requirements of HP's or IBM's newest blade (10U) chassis fully stacked, how many of them can you fit in a standard size rack (42 U) ?

10U rack chassis, 42U of space, you can fit 4 of them in a rack with 2U to spare.

Is this is a good idea? That depends a lot on your infrastructure. If you're using passthrough ports instead of integrated switches that could be up 512 cables that'll need to be run to another rack for connecting to things, as well as the power to drive it all. Using the integrated switches makes it easier, though getting power to it will still take some creative routing.

I have some 50U rack with 5 x HP C7ks in them, so long as you can power and cool them there's no issue.

We have 12 C7000's fully loaded with BL465 G6 servers and using virtual connect modules installed in 3 42u cabinets located in a colo facility. We have 2 50Amp 3-Phase power circuits going to each rack. Each circuit connects to a HP 0u PDU. The PDUs then feed power to the enclosures. We were lucky that this was a new installation and we didn't have existing racks which needed to be utilized. This allowed us to order the enclosures pre-racked and cabled to the PDU in HP cabinets. We got HP wide racks with cable channels on each side. This has worked out great because the enclosures otherwise leave little space for cabling in the cabinet. With multiple enclosures connecting to redundant ethernet and fiberchannel switches the cabling adds up fast.

The colo facility cooling is pretty traditional underfloor cooling with alternating cold and warm aisles. We have 4 rows of 5 racks in our cage. We also have a 3PAR T800 SAN which throws out a good amount of heat so we had to work closely with the colo facility regarding placement of the equipment to optimize cooling. Basically we have a C7000 rack in 3 of the 4 rows and the 3PAR in the 4th