They are not regular PCI slots. The are much smaller than a regular PCI slot, the only thing I know of so far that uses them are some motherboard makers have their onboard sound in them instead of a chip built into the motherboard.
The PCI-E interface will eventually accomodate all the current PCI interface cards you see right now (eg. network, sound cards) and eliminate PCI altogether. This situation is no different between ISA slots being eliminated with the birth of PCI cards.
If you want to see a card which will most definately use the PCI-E interface on release, see this.