Planetary onslaught came out this past weekend. My wonderful girlfriend picked it up for me as an early Christmas present.

I was reading through it and I have to say
GW done a good job bridging planet strike, stronghold assault, and cites of death into a nice campaign format. They broke and campaign into 3 phases. (Planet strike, stronghold assault, and cites of death) You have to play the campaign in order, because at the end of each mission, the winner gains a special bonus to be used in the next game. The winner of each phase gains a bonus that applies to the next phase of the campaign. There is also some character development with your warlord.
For everything Planetary onslaught brings, it also is lacking in some areas. For starters, not all the fortifications are in the book. 3 that I noticed were mission was Plasma Obliterator, Plasma Obliterator Aquial Strongpoint, and the new fortification that is the plasma reactor. Also the campaign is designed to be played in 1v1 fashion. You may be able to adapt it to be a small group campaign, but you will have to play with the rules a bit.