If you previously installed, ran, and created a new account using the geth client and then installed Mist / Ethereum Wallet, your accounts from geth typically show up in Mist / Ethereum Wallet. This is because the keystore files (your encrypted private keys, in JSON format) for accounts created via geth command line and Mist's GUI are stored in the same place.
However, on Linux, the folder may have changed in the past few months. While researching this answer, I found some conflicting reports of where accounts' keystore files were saved on Linux.
In addition, if you were using the eth client (C++, cpp-ethereum) instead of geth, that may also result in accounts being in different places.
Hypothetically, all keystore files are saved at ~/.ethereum/keystore on Linux, but it seems like occasionally this isn't the case.
So, in order to get the account you previously created via geth into your Mist, first locate where your Mist is storing the keystore files. Go to the menu bar and select "Accounts" -> "Backup" -> "Accounts". This will open a folder.
If you already know where your geth keystore files are, or you have a backup, simply copy that keystore file into the folder you just opened via Mist and Mist will read it.
If you have no idea where the keystore files are, you can try doing a search on your computer for either utc--2016 or utc--2015. The full filename for the keystore files looks something like: UTC--2016-04-18T03-24-17.849Z--8688b2e109b93a77578eea43b3ebf6bc4be996f5.
I can confirm on my Mac that searching for utc--2016 displays all my keystore files that were created in 2016 that are not in the (hidden) default location for the keystore files. Explicitly searching the hidden (on Mac it's referred to as "system files") returns search results for the keystore files Mist is reading.
It is planned to be released at a later time, however for testing purposes, a developer release was made in November: https://github.com/ethereum/mist/releases/tag/0.3.6
It is not compatible with Homestead, well at least the included geth/eth certainly isn't.
Best Answer
Mist DApp Browser
Mist is the browser for decentralized web apps. What Mozilla Firefox or Google Chrome are for the Web 2.0, the Mist Browser will be for the Web 3.0 (which will be decentralized).
Mist is still in heavy development (for instance it's not recommended to visit untrusted DApps until the full security audit is done). You can find the releases to date here. The current release allows you to open any Ethereum DApp with the Mist Browser (with the disclaimer above).
Ethereum Wallet DApp
All other releases of 'Mist' are no Mist releases at all, but a bundle of Mist Browser with a single DApp: The Ethereum Wallet, also known as the Meteor DApp Wallet.
These releases are therefore called Ethereum Wallet as it only offers a bundle of the Mist browser with a single DApp: the wallet.
The future, with Metropolis release, will provide a full Mist Browser able to open any DApp available out there. The Ethereum wallet will only be one among them.