The USB key that HP offers for integrated hypervisors inside ProLiant servers uses single-level cell (SLC) NAND flash, instead of the more common multi-level cell (MLC) technology. Why SLC? Because not all all USB keys are created equal. An SLC-based key will last about 10 times as long than a cheaper MLC-based USB stick you'd typically find in computer stores.
You don't need an especially reliable key if you're just using it to move software or install device drivers. However, there are some good reasons why people install USB keys as a permanent boot device for integrated hypervisors like VMWare ESXi (Simon Seagrave describes the reasons on his Techhead site).
SLC (Single Level Cell) and MLC (Multi-Level Cell) are two types of NAND flash memory usually used in USB keys, with MLC being much more common.
SLC technology works by storing a single level of voltage (charge) in each memory cell. That charge represents single bit of information. If the cell is charged, it represents a "1"; if the cell isn't charged, it represents a "0". Each cell holds 1 bit of data.
MLC technology stores one of four different charge states in each cell. So each cell can either be uncharged, charged a little, charged a lot, or charged full. Four different possible states ("00", "01", "10", and "11" in binary) means each cell stores 2 bits of information.
So MLC has a cheaper cost per bit. That makes it more popular with consumers, meaning more memory makers produce it...and supply-and-demand end up making MLC a lot cheaper than SLC.
However, the two technologies react differently to the biggest problem with NAND flash: wear-out. NAND flash cells, whether SLC or MLC, can only survive a certain number of writes (changes to the charge state) before they degrade. However, SLC can tolerate around 100,000 write cycles, while MLC can only endure 5,000 to 10,000 cycles. So if you wrote to a single cell once every hour, an average cell in an MLC-based key would fail in about 1 year, while you'd get about 11 years of life out of an SLC-based key.
Now, a copy of ESXi actually doesn't do very many writes to its 'host' key. When ESXi boots, it sets up a RAM disk in memory and mostly uses that, even for log files. There are also usually two bootable images on each key, providing some redundancy; plus, if a key fails, it's relatively easy to restore the host to a new key. All that being said, it's pretty rare in the data center to find a way to spend about $70 more to get a device with 10x the reliability.