That issue exists as long as UAC itself does exist (Vista). If it stays in the Taskbar and blinks, it's a result of the "Don't steal focus" feature (you probably interacted with the window shortly before). But there are cases where UAC unobstrusively opens in the background (where you can reach it with ALT+TAB), or even behind modal windows (where you can't). That's called a bug, a very longstanding one. Maximum annoying, more so since often, it doesn't get a Taskbar button of its own.
There's another one: With multiple Explorer windows open, the wrong window is brought to the foreground after confirming UAC.