Mounting these things upside down is done all the time, generally to keep from running the service entrance conductors from the bottom of the panel to the top. As Paul pointed out, as long as the service disconnect is not upside down, no problem.
You need to forget about line being on top and load being on the bottom, this could be a dangerous assumption For example, a 200 amp breaker with lugs on both ends could have the power coming in the bottom. This would be allowed as long as there are no Line and Load markings on the breaker or the enclosure. You turn off the breaker & assume the lower lugs are de-energized & you could become a statistic.
Besides which, when you order what many of us consider a real panelboard, not the cheap load centers we usually deal with, you specify if you want top feed or bottom feed.
[This message has been edited by Tom (edited 07-29-2003).]