I asked David Peterson about this in the latest IRC chat. Here is a transription of the part that discussed the word thank you:
[2011-04-10 22:12:21] <ingsve> we had a question on the forum about the comment in one of the clips that the dothraki have no word for thank you
http://forum.dothraki.org/index.php/topic,36.0.html[2011-04-10 22:12:38] <ingsve> is that true, and if so is that from the books or is that your choice?
[2011-04-10 22:13:34] <DavidJPeterson> You know, that wasn't in the original script.
[2011-04-10 22:13:45] <DavidJPeterson> They added that based on something *I* said in the original press release.
[2011-04-10 22:13:58] <ingsve> ah ya, that's true
[2011-04-10 22:14:44] <DavidJPeterson> I remembered the first time I saw that (Jorah says it) thinking, "Wait a minute, I don't remember that..."
[2011-04-10 22:15:04] <DavidJPeterson> Then it hit me, and I realized, "Oh, hey. They picked that up from me!"
[2011-04-10 22:15:56] <DavidJPeterson> Then I forgot it and created a word for "thank you", anyway.
[2011-04-10 22:16:04] <Jenny> Hah
[2011-04-10 22:16:08] <ingsve> hah
[2011-04-10 22:16:24] <ingsve> what was the original thinking behind not having it?
[2011-04-10 22:16:45] <DavidJPeterson> The Dothraki practice of not trading.
[2011-04-10 22:17:09] <ingsve> ah, so I basically guessed right in my answer to the question
[2011-04-10 22:17:26] <DavidJPeterson> I don't think it'll actually be used in the show—and if it is, it'll be used by non-native speakers.
[2011-04-10 22:17:27] <DavidJPeterson> Yeah.
[2011-04-10 22:17:42] <ingsve> also they seem to have a sense of entitlement in that they see what they want and take it rather than getting it by conventional means
[2011-04-10 22:17:45] <Lajaki> Hopefully that's the only time that'll happen.
[2011-04-10 22:18:06] <DavidJPeterson> Right.
[2011-04-10 22:18:28] <DavidJPeterson> It's still a good expression, I think. It doesn't necessarily need to be used as "thank you".
[2011-04-10 22:18:31] <DavidJPeterson> Though it can mean that.
[2011-04-10 22:18:53] <DavidJPeterson> Literal is "much respect to you".
[2011-04-10 22:19:28] <DavidJPeterson> If there was some expression one would use as "thank you", I think that'd be it.
[2011-04-10 22:19:37] <ingsve> ah ok
[2011-04-10 22:20:08] <DavidJPeterson> Though it can be used entirely apart from any gift giving (and probably would be).
[2011-04-10 22:21:28] <ingsve> so the expression is something involving athchomar shafkea?
[2011-04-10 22:21:46] <DavidJPeterson> Or yeraan.
[2011-04-10 22:21:50] <DavidJPeterson> Depends who you're talking to.
[2011-04-10 22:21:53] <ingsve> ah ya
[2011-04-10 22:22:01] <DavidJPeterson> Multifunctional. Like "aloha".
[2011-04-10 22:22:43] <ingsve> is there a word for "much" in the expression as well?
[2011-04-10 22:23:05] <DavidJPeterson> I thought I'd already given that one to you in one of these chat sessions...
[2011-04-10 22:23:12] <DavidJPeterson> Yeah, there it is.
[2011-04-10 22:23:15] <DavidJPeterson> It's in the dictionary.
[2011-04-10 22:23:19] <ingsve> sekke?
[2011-04-10 22:23:22] <DavidJPeterson> san
[2011-04-10 22:23:44] <ingsve> ah we only have that as heap or pile
[2011-04-10 22:23:49] <ingsve> as a noun
[2011-04-10 22:23:59] <DavidJPeterson> It is a noun in the expression, actually.
[2011-04-10 22:24:10] <DavidJPeterson> San athchomari yeraan. (Or shafkea.)
[2011-04-10 22:24:20] <ingsve> ok
[2011-04-10 22:25:06] <ingsve> sort of like saying piles of respect to you
[2011-04-10 22:26:03] <DavidJPeterson> Right. Though understood conventionally.