The new iPhone SDK requires that developers upgrade to OS X Leopard, which is a nice excuse for most of us to drop 100 bucks on a new operating system that does, err, exactly what the old one did. I am sure I'll come across some amazing new feature that I couldn't live without soon enough... Obviously there is the addition of TimeMachine, but funnily enough I was already doing backups.
One thing I was expecting was a seamless upgrade. I was sorely disappointed. The first complaint from the Leopard Installer DVD was that my disk had the wrong partition map. I upgraded my macbook's hard drive a year or so ago, and apparently had selected APM instead of GUID when I installed. To fix that, I followed the instructions at http://www.macosxhints.com/article.php?story=2007102511133285 which took several hours (LOTS of copying data to/from external drives). This involved the use of an excellent program called Super Duper which I am happy to report does Just Work.
The next complaint was harder to resolve. The verification process failed half-way through installing. Skipping the verification resulted in a failed install (not on my main drive, on my backup!). Googling revealed that apparently this problem was not uncommon, and was related to after-market RAM upgrades (that's me) - though doing a full RAM soak test revealed no problems at all. I eventually managed to install the thing by restoring the DVD onto a partition on my external drive, and then booting from that.
The iPhone SDK was, fortunately, easier to install!