In my most recent [single player] game of Sins, I managed to create quite a large profitable empire, but found myself getting overwhelmed as my opponents started attacking multiple planets at once. Infact, before I knew it I'd lost 3 planets at once. This may partly be due to my inexperience at the game, but I also think the game both lacks methods of informing the player as well as perhaps giving them information overload.
In this game I was getting inundated with attack messages (quite a few of the times it was for a single ship that had arrived in a gravity well belonging to one of my planets, and sometimes the AI even believed this single ship could get past my defences and sent me a taunt!), so some got missed and some got ignored because I either didn't think they were a threat or they got buried underneath the rubble.

My suggestion is this: In addition to the Report dialogs, when a planet comes under attack, could its icon (both the one on the empire tree and the icon that replaces the planet when you are zoomed out) flash to indicate that it needs attention?
I think it would be best to have the icon flash the colour of the player doing the attacking, as simply choosing a colour like red, while a typical "warning" colour, could obviously already be in use by the player (or an opponent) as their standard army colour. Alternative you could flash a symbol either on or next to the icon that tells the player their attention is needed.
You might be able to expand this to work for circumstances other than being under attack, and that way a player can zoom out and see exactly whats going on.