Given its low power requirements and persistent screen state, E-ink makes for a great wall sign, but it's still comparatively rare (aside from a handful of products in the signage industry). Rather than wait for commercial solutions like Vikuara's BT-enabled screens, one industrious hacker built his own 6" E-ink sign which can be refreshed over Wifi.
A hacker going by the name Sprite_tm documented the steps they took to turn a stock 6" E-ink panel into a wall sign suitable for posting project updates.
Sprite_tm bought a bunch of screens online (they're available on Amazon), and proceeded to scratch build an interface board. When that proved less than successful due to mismatched part numbers, the hacker bought an old dead Sony Reader off of Ebay and reverse-engineered its screen interface protocols.
if that sounds like a lot of work, it is. But on the upside Sprite_tm learned enough to overcome the problem he had had before, enabling him to show this:
The resulting sign uses a custom driver board and is powered by an aftermarket battery for the Samsung Galaxy Note II.
If you'd like to build one of your own, Sprite_tm says that all of the work has been released under an open-source license:
The pcb and the lasercut designs are licensed under a CC-BY-SA license. The firmware for the ESP8266 can be gotten by cloning this git repository. If you use any of these things, I'd love to hear about it!
But if you don't want to do things the hard way, you could always start with an ereader and modify its software. I know of a couple projects which have turned a Kobo Wifi and a Kindle into displays for weather stations.
I don't know that very many people will need a weather station, but those projects are quite close in principle to Sprite_tm's project. They just started with commercial hardware, but ended with Wifi-connected displays which function like remotely controlled signs. With a little modification those projects can be adapted to serve another purpose.