There is the Eyefi product line which basically wraps a small wifi chip together with a small arm SoC into an SD package with some limited internal storage.
They do work but the range is rather limited and you have to use their software in order to use them. Also only SD cards in their product line which aren't used in most mid/high-end DSLRs.
The SPI bus used by SD cards makes this whole thing quite a lot easier than with the old CompacFlash interface. Though the CompacFlash interface can act as a small form factor version of the old P-ATA.
In theory it's not all too hard to make and adapter with off the shelf parts, there is a huge discussion about this on this forum: http://www.magiclantern.fm/forum/index.php?topic=5470.350
It seems they never came to any solution but then again most of them are technology enthusiastic photographers. You will need some knowledge about electrical engineering to pull this off.
EDIT: Seems there are adapters for SD to CF: