Mo and Troy of Travel 2.0 blog have started an excellent discussion of the use of Twitter in the travel industry here. The focus is mostly on how companies can speak to customers but with the use of hashtags, it's an effective way of sensing market sentiment.
For example, by goomg to hashtags.org and search under Ryanair, I leaned that
their flights may be cheap but in the event of a drop in air pressure reach for your wallet:
!
