Ben Horowitz, one of Netscape's founders, presents some solid analysis of why we are not in a bubble. Some will disagree, but the article is fact-based and worth a read:
While we can see many signs of a bubble these days, it's important to keep in mind that signs of a bubble look almost exactly the same as signs of a boom. In fact, it's usually not a bubble until everyone agrees that it's a boom.
Update: Reader Martin Seebach adds:
@swombat Another point to consider: No bubble ever looked like the last one. Learning from the past isn't about waiting for a repetition.