MTBF for power supplies

Hope this isn't too far off topic...

  We're looking to beef up some of the equipment in our data
center's core backbone and one of the topics we're looking at is
redundant power supplies (specifically for Cisco 7000/7500 chasis's).
Cisco claims that the "observed" (vs. design - which I don't know) MTBF
is 2,000,000 hours for these supplies. Without much to back this up,
this number seems awfully high. Does anyone have any pointers to MTBF
numbers for network equipment (not necessarily Cisco related) and/or
any of their own experiences to relate? We have had one power supply
fail already (I don't know the MTBF but certainly an order or two less
than the number Cisco gave) and at that time, we pulled a supply from
another box. At the moment, we don't have enough of whatever (spare ports,
extra supplies, etc.) to do that anymore.

  Any info appreciated. Thanks.