OS X Lion boots to 64-bit by default

Yes, it does, as long as your machine is 64-bit capable which basically means as long as it's not too old or, more particularly, that you have one of these processors: Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor.
My late-2009 MacBook with a 2.26 GHz Intel Core 2 Duo boots to 64-bit with Lion.
To see if you're running on 64 or 32-bit go to the Apple menu (top left)>About This Mac>More Info…>System Report…>Then go to Software in the left pane, and then check the 64-bit Kernel and Extensions: in the right panel which will say either yes or no. Yes means you're running in 64-bit and no means 32-bit.
If, for whatever reason, you want to run Lion in 32-bit although your machine can handle 64-bit, just hold down the "3" and "2" keys while booting up until you see the Apple with the spinning wheel.
Should you have any problems getting to 64-bit mode, you might find this utility useful
So what's the big deal about running in 64-bit mode. Essentially, it means 64-bit mode can handle much larger binary numbers than 32-bit mode so complex operations should be considerably faster.
In 64-bit mode, however, apps that are not optimized for 64-bit operation may stutter.
I spent some time playing around with Lion in both 64-bit and 32-bit mode and, I have to say, I really did not notice any significant difference at all other than the browser I had adopted in Snow Leopard (OmniWeb 5.11) crashed quite a bit in 64-bit. Seems we have to wait for OmniWeb 6 until 64-bit compatibility is available.
That's a pity as it's a great browser.
So, right now I'm trying out both Safari 5.1 and Stainless 0.8, both of which are Lion-ready.
I honestly have never liked Safari and find the inability to populate the bookmarks bar with favicons just too distant from my present notion of an ideal browser, despite the fact that Safari does seem very sprightly in the little testing that I've done. Maybe I just need to get used to it.
On the other hand, I've liked Stainless for some time and the current version does not disappoint. It's clean, relatively light and does everything I want without all the baggage that Firefox has.
In addition, neither of these two browsers crashed even one time for me.
Finally, here's a brief article about using 64-bit mode in Snow Leopard which basically concludes that not too much can be expected right now.