I did one of these for the World Series, and with the NBA Finals soaring of late, this seems a nice thing to look at. The following chart looks at the last six years of NBA Finals ratings, game-by-game. I was going to only do the last five years but decided to throw in 2006 since that was another Dallas/Miami series. Sorry if that clutters things a bit!
I left off a few years that I have access to just so it wouldn't get impossibly cluttered, but I should note that 2004 was the first year the NBA switched to the Sunday/Tuesday/Thursday scheduling of the present. Prior to that, it looks like the games were scheduled on Wednesday/Friday/Sunday. As with everything sports (and non-sports), the Friday games were weaker than it seems like they would've been for those corresponding games on a level-HUT playing field.
The official 18-49 numbers for the 2011 Finals: 6.5 -> 6.5 -> 6.1 -> 6.7 -> 7.4 -> 9.7. So how's that stack up among recent history? Well, pretty well. I remarked at one point that the show was only barely above 2010 and that might be considered a little underwhelming considering how loud the buzz was around this series. But looking at more years of ratings, it's clear that 2010 was also one of the stronger series in recent years, and it's also clear that most series without the Lakers don't do that well. So I'm thinking posting Lakers-esque numbers really was a nice accomplishment.
The first Mavericks/Heat matchup in 2006 was probably the second-weakest of the last six years, while LeBron's first Finals in 2007 did pretty dreadful numbers (all four games between a 3.4 and 4.0 demo). 2003 (Spurs/Nets) and 2005 (Spurs/Pistons) are a couple years not pictured that also struggled in the 4.0 vicinity. Moral of the story? Spurs are ratings kryptonite, I guess.
Probably the last series to do better than 2010 and 2011 was the 2004 series between the Pistons and the Lakers, which didn't hit the heights of the last two series but started off really well (the first three games all scored a 7.0). Its only problem was that it was a short series, but the 9.1 demo of Game 4 and the 9.4 of the decisive Game 5 suggested it probably could've gotten up to the decisive game numbers of the last couple years if it'd only lasted longer. Having a Game 7 is always nice. (2010's 28+ million viewers and 11.4 demo for Game 7 towers over all other recent Finals results.)