(I'll post a photo of the shirt design first, followed by the cake entry.)
I'm in love with this design titled "Alice's Sewing Basket:"

I want this as a statue on my desk for all of eternity.
Here's one called "Firelight Cottage."

Look at all that detail! I would never want to cut into that robot. EVER!
This shirt is too cool: "Doc Franklin's Miracle Tonic"

But Neli's cake is even better:

You can see closer detail shots at the link. So cool.
This design is called "Long Journey." I love the swirly blues and greens!

Vina did an awesome job of bringing the design to life:
Gorgeous!
"Strange Birds" is one of my favourite designs on Threadless. Look at these colours!

I love the white chocolate pretzel barrier!
By Jamie Hoffman
And the rainbow pools are chocolate ganache mud cakes? Yeah. I'm so there.
"Diver No 12" is such a neat design on its own...

And the rainbow pools are chocolate ganache mud cakes? Yeah. I'm so there.
This shirt called "Extraordinary Observer" is really cool:

But Georgia's cake really blew me away:

The base is made from cake, and the detail above is all pastillage and gum paste. Very impressive!!
I'll wrap up with a few favorites from the 2D category.
This sweet design is called "A Vehicle Built for Dreams." It reminds me of books I read as a kid:
This sweet design is called "A Vehicle Built for Dreams." It reminds me of books I read as a kid:

"Sugar Land" is the perfect design to turn into a cake:

"Birdsong" is a very intricate design:

It took her a whole week to make! Well worth it, Megan.
There are HUNDREDS of entries on the Threadcakes site, so be sure to check out the gallery for more! This year's winners haven't been announced yet, but we'll let you know when they are.