For people with the beta client:
"Q: Do I need to delete the beta client and download a new client for launch?
A: No, you do not need to delete your beta client or perform a new download and reinstall. If you have the live beta client, you can simply patch it up to the launch build starting on Thursday the 27th of March. We encourage you to patch up to the launch build early so you’re ready to go for Early Access or launch day!"
For people without the beta client:
Q: I pre-ordered/pre-purchased the game, and don’t have a beta client. When can I start downloading the game client?
A: Once you complete the pre-purchase from the ESO online store or enter a 3 / 5 day Early Access pre-order code via your account page, simply log in to your account at https://account.elderscrollsonline.com/ from Thursday 27th March onwards and follow the prompts to begin downloading the game client. We encourage you to download and patch the client early.
Link to these posts: http://elderscrollsonline.com/en/news/post/2014/03/25/early-access-and-launch-faq
--No update as of yet, am sitting here staring at my launcher. Anybody know anything different as to what time they will allow us to start updating to the official patch?