The Notes client constantly steals focus from other apps while the application starts, and when opening databases. This is quite annoying, as opening Notes for the first time takes a while, and so does opening databases over a remote connection.
The current behavior goes against MacOSX normal behavior, and stops any attempt at multitasking while Notes is stalled loading or waiting for data.
It pops up at least every time a new email comes in (and I have it set to alert only by sound when a new mail comes in). I also think it pops up when a replication goes off. It is extremely annoying.