I think some of the comments here have been overly harsh.
I know some people were a bit shocked when they realized it wasn't Steve. Like you, i could tell something was off - either Steve had a (oh dear) "frog in his throat" or someone was taking his place...and once Kermit handed his cd over, i knew it was the latter. Considering Steve wasn't at the Daytime Emmys this weekend either, chances are he's probably sick or something and we should all be sending him positive energy and best wishes.
Having said that, i think the Muppets and Steve's understudy did wonderfully considering the circumstances. This was obviously something that was arranged before Steve had to bow out for whatever reason and the team should be commended for going on with the show. They probably did a bit of tweaking to where Piggy/Eric carried the performance (and dropping Statler/Waldorf - though since Steve himself is a fairly recent replacement for Statler, they probably could have had someone else perform him this once). Whoever did Kermit (i'm also guessing Rudman) was terrific GIVEN THAT IT WAS OBVIOUSLY A SUDDEN EMERGENCY RECAST! Yes, it wasn't up to the standards we would expect if this was some kind of planned performer change that was in the works for quite some time now and if this was going to be something more permanent. Sub-Kermit was no doubt thrown the role on very short notice and up to that point had probably never thought about or imagined playing the frog. I mean, Steve himself had practically a year to mentally and physically prepare himself for the challenge between the time the decision was made for him to carry on the mantle and before he made his first major public performance.
Consider this forum is made up of the "die-hards" - those who are apt to notice. The performance was strong enough to where i'd say the vast majority of people watching (the general public at large and the less hardcore base of casual Muppet fans that enjoy them but don't follow fansites or post in forums) didn't have a clue there was a sub tonight. I say whoever filled those almost-unimaginable to fit into flippers did admirably and we should be THANKING him for saving the show. Whether it's with the Muppets, any other puppet troupe, or stage or screen human actors, the job of the understudy is a thankless one. They work as hard as the person whose role they're understudying in the case an emergency arises and live with the double standard of never having their work seen or appreciated or if they do get the chance to fill in, they're looked down on as "just a substitute for the real thing" (as someone who for a couple years was one of the go-to guys for theatre directors to call when they had to do an emergency recast close to showtime for plays since i earned a reputation as someone who could learn lines and take on a part very quickly after i took on the second male lead part in a production of Edward II i was assistant directing after the actor came down with jaundice (sp?), i know about this well!)
Now maybe about three or four years ago when Disney looked like it wanted to start having a whole bunch of possible Muppet recasts for cruise ships, minor appearances or just plain emergencies (leading to the actually surprisingly successful Save the Muppets fan campaign) i would have been worried that this was a more conscious thing. Until i hear otherwise i'm sure that something happened where Steve just couldn't appear (much to his own regret i'm sure) and the whole team rallied together (very much in the spirit of the Muppets) and not only allowed the show to go on but worked together in a way to tailor the performance around the challenges they were facing to where damages were minimal and the overall performance itself was Muppety Goodness.
Not to mention, i think it says a great deal when Steve has performed Kermit long and well enough now that fans have come to accept Kermit as Steve and vice versa as opposed to "Steve's Kermit" to the extent that we get a tad huffy if someone isn't Steve-level Kermit. I quite liked how the sub both sounded very much like a cross between Jim and Steve's interpretation.
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I guess that's what the married life will do to you. I never tire of my Muppets. It's great that I watch them with my kids and they really appreciate the humor like I did when I was their age. 
