There are some concerning issues still with the US economy which could effect equity valuations. Company earnings are good, but we tend to base valuations on future performance nowadays. Some nice threads in the 'economics' section about this.
The last 12 months rally after the Iraq-conflict buildup (with some exhausting losses for many) can be seen by some as a "revenge trading" rally, especially when considering the traumatic dot-com collapse as well.
I think we are definitely in for some real profound change over the long haul. However, we're much too nice to have an off-the-cliff drop right now. See-sawing is always much nicer. On the other hand, Al-Qaeda could contribute to a change in that respect.