I don't believe the icons have always been that way. They've changed a bit over the months and years, though they've certainly been this way since about December, IIRC. The other icons either had no dragon on the border (the border was just a line), or didn't have a "metallic" dragon on the border. Of course, that doesn't mean anything on the face of it.
While WotC has made some... dubious business decisions in the last couple years, I can't believe they'd be so dumb as to put the Compendium in the intro package and not the Character Builder. The Compendium is the obvious pick for a "top tier sweetener." I think you're reading way, way too much into some web designer's aesthetic choices.
No one's said THIS IS GOING TO HAPPEN OMG EVERYONE WATCH OUT!
It's a curiosity I noticed, and something of a reason to talk about what you'd like/expect to see in a tiered pricing scheme, if one were to come out of WotC. I bet it's at least something they've been bandying about, whether or not it's something they're seriously considering (or already have a vague plan for implementing).
If they did go with "compendium < character builder + mags < monster builder" motif, I could see the line being the amount of tech/money investment WotC had to make to get it right, and how "digital realm" the offering is. The Compendium is a basic data entry project, and aids anyone with a laptop at the table for rules look-up. You can't very easily do much with it other than look up rules, though (and, currently, you can't even look up all the key rules, like the fighter's combat challenge). The character builder and the mags are significantly greater cost to WotC, and are a more robust "playing on the web" thing. You make your character on the computer, you're spending more time at the thing thinking about D&D than you are if you just hand-crank your character. The Adventure Tools, of course, reflect the highest investment in using webtools.
Personally, I would bet on the tiers being content-driven rather than app-driven. Something like, lowest tier gets the Essentials books; middle tier gets all published books but not Dragon/Dungeon content; top tier gets the whole shebang.
Why would they do that, when the biggest selling point for DDI so far has been "all of your information, and tools to help you manage it?" Better to charge more for more tools than to charge more for more info, I'd think. But we're rampantly speculating so your idea has as much merit as mine, I guess.
Why are we all talking about DDI tiers suddenly, anyway? Seems like every week there's a new thread on the subject.
You didn't HAVE to click on the thread. It says right in the title it's about DDI tiered pricing. You didn't have to post in it if you're annoyed by the subject matter. If you don't want to talk about it, posting in the thread and bumping it is a strange way to discourage conversation.
Also, if you'd like to link to a few of those hundreds of other threads, I'm sure the mods could merge 'em and make the world an easier place for you, if there's too many. I did a quick scan of the first few pages and didn't see one relating to tiered pricing or the icons possible hint at all, but I certainly could have missed it.