I had grown accustomed to using Command-Tab and the Right- or Left-Arrow keys to switch apps in 10.4, but that functionality changed a little in 10.5.

In 10.4, holding the Command key, tapping Tab, then using the Left Arrow twice, one could get the end of a (potentially long) list of running apps -- the first Left Arrow press would select the 'previously used' application (the leftmost icon in the Command-Tab bar), and the next Left Arrow press would jump to the icon at the far right of the Command-Tab bar. Basically, if the leftmost app icon was highlighted, hitting the Left Arrow would highlight the app at the right end of the list, and vice versa. 10.5 doesn't allow this wraparound feature; pressing Left Arrow with the leftmost icon highighted does nothing at all.

The solution? It was hidden in this older hint: after pressing Command-Tab, release Tab (still holding down Command) and then press the backtick/tilde key (above Tab on the US keyboard -- this key may differ in other regions of the world). The backtick/tilde key, along with the Tab key, doesn't care about the new "hard stop" at the left edge of the Command-Tab switcher, and will jump across that boundary to the rightmost application icon.

[robg adds: As noted in the original hint, you can also press the End key to jump to the rightmost entry in the Command-Tab switcher (and Home to get to the lefmost entry). I don't know why they blocked the wrapping of the arrow keys, whether it's a bug or a feature. If it's a feature, expect that the Command-` shortcut will also stop working this way in a future update.]