Disclaimer: obsessing over Diamond Sales Figures is bad for your health, bad for comics, and most likely contributes to global warming.
Lost in all the hubbub over the fact that Final Crisis #1 fell short of Secret Invasion #2 (though I'll get to that in a moment...) is that fact that the first 2 chapters of "Batman R.I.P" sold surprisingly well, right up at the 100,000 mark. Whatever disappointment DC might be feeling over Final Crisis numbers should be tempered a bit by this fact.
I'd chalk up the good sales on "Batman R.I.P." to the confluence of: Batman fans + Morrison fans + Tony Daniel fans + Big-ish Event fans + pre-Dark Knight movie excitement. "Batman R.I.P." could trn out to be this year's "Sinestro War"; maybe DC will learn that these minor-sized big-ish events will work for a better long-term strategy than the massive Crisis crossovers.
The other big sales surprise was the strong showing for Avengers/Invaders #1, proving that just about anything that Alex Ross is involved in is Direct Market gold.
As for the Final Crisis numbers: it's debuting at about 100,000 units less than Infinite Crisis did, which has to be disappointing. This is the first time that Marvel & DC have had their big event crossovers go head-to-head, so there was bound to be an effect. (We should also note that FC came out the last week of the month, so there are no re-order numbers included in the total, while SI came out the first week of the month so there is like a boost from re-orders there.)
It will be interesting to see the June figures: will Final Crisis take the same size drop that Secret Invasion did? Will Batman remain strong? How will Trinity do? (I think it was a mistake to start the weekly Trinity at the same time as Final Crisis; they each draw attention away from the other, and an eight-month break between weekly series was warranted.)