WRT the "psychological boost" / "momentum" business, good points have been well made in several of the above posts.
But the people who place faith in "psychological momentum" and suchlike are the players and the ex-players, who talk about it all the time as being highly significant. Are they imagining it? I find it hard to believe that they are. From my own much less vaunted experience of playing club cricket, I think that the mental state with which you enter a game is hugely important to how you perform, and your recent results play a big part in shaping that.
The fact that "momentum" doesn't always produce results pointing one way, or results that are entirely predictable, doesn't mean that it doesn't exist. We can all point to teams that have bucked the psychological odds, but it's easy to overlook the examples where they didn't. It happened to England teams in the Ashes for years, as is acknowledged by all those who played on both sides. For instance the 2006/7 series, post-Adelaide, was slaughter because one team was mentally broken and the other was mentally ****-a-hoop.
Now I'm not pretending that the draw at the Gabba 2010 was comparable with the holocaust of Adelaide 2006. Except perhaps for Village Mitch who, if he plays on Friday and starts to do badly again, may well (to use an expression which some optimistic Aussies were using before the series began with regard to the English) find old scars opening up again.
And finally... I recognise that all the above might be complete crap. Any cricket fan, and any cricket player, will tell you with complete certainty that "form" exists, and that it is a powerful force, but statistical research tends to show that it may be completely illusory. Research on baseball players' "hitting streaks" has shown that they basically don't exist, but rather are a creation of our innate tendency to see patterns where none exist. Likewise someone on CW did a quick analysis a year or so ago, taking Tendulkar as a random example, showing that his score in one innings was effectively useless as a predictor of how he would score in the next. And just as "form" might be an illusory concept, so might "momentum".