He's got two good starts under his belt now. You have to consider that:
a) He's now back in the minors- I mean, the National League. That's helping; he's got a basically given out at least once through the rotation instead of a career hitter.
b) He's not playing in the AL East in particular, which is a nasty division.
c) He's still got stuff, and now he doesn't have a baBIP of .386, as he did in Boston. Some of that was surely that his slider wasn't as crisp as it had been, his splitter wasn't working quite as well... but you get shelled on mistakes harder than anywhere else in that division, so that's rough, and it was a comparatively small sample size; he pitched only 40 innings for Boston and still had some good peripheral numbers.
As ssk99 said, his off-speed stuff is looking nastier in St. Louis, though.
What else to say... he's had a LOT more horizontal movement on his fastball in St. Louis, like 2-4 inches of extra tail. Noticeably more sink on his curveball in his last 4 starts. Got the big bite he used to get out of his slider against the Padres, and though it was a little less in Game 2, it was astill working well enough compared to his fastball, something like 8 or 9 inches difference. Nice big difference in vertical movement between his heater and his slider/change combo, too.
Pitchf/x is annoying sometimes for pitch classification and doesn't show his splitter (even though the ptich type chart on fangraphs does), and since the man himself has been talking about his splits, I'm going to assume that they bleed over into his slider/change, since that's about the velocity they'd be at, since it's about the same speed as those two pitches.
In any case, if you look at THOSE two starts compared to his first start with the BoSox, which was June 25th against the Nationals where he got shelled, you could see a couple of differences.
One, he wasn't really throwing the curveball well, not getting the same sink out of it. Two, he wasn't mixing the speed on his heater as well. Three, his change/split/whatever wasn't sinking as well.
Bump it up to his start against the Rangers.
He didn't have good velocity on any of his pitches that game, and that was a big deal.
Last start for Boston, against the Yankees. Wasn't getting as much tail out of his fastball and change, his release point wasn't as consistent (in fact, it was all over the place), velocity wasn't great...
It's not a lot to go on, but you know that he got some confidence back from the Padres game, and then again from the Nationals, and you could see he had a little more zip in his fastball, a little more bite on his slider in some games, he was getting better movement on his pitches... He's old, he probably needed the time to stretch himself out or something, right? He IS still trying to make a comeback from surgery for an injury issue that killed his 2008 season, and he IS 42 years old, which isn't the most resilient time in a person's life, especially after 20 years of professional baseball.
It sucks for Boston, but whatever, right? Buchholz looks really good, Lester and Beckett don't suck (heh), I mean, the rotation's fine.

Maybe Dice-K will get himself together, and it's possible that Wake will come back and make 5 or 6 starts to end the season, right? So it's not like the absence of Smoltz is killing the Sox. It would have been nice, and it surely is for his bank roll, but some of that resurgence is surely league-related; I doubt he'd have tossed those two games quite so successfully here. There's also the influence of Dave Duncan, who isn't there in Boston. There were some issues with pitch-tipping maybe, and some mechanical stuff. Duncan is the guy who worked with Dave Stewart and Dennis Eckersley in Oakland, so it's less than surprising that Smoltz is working out a little better there, even from subtle adjustments.