In terms of BluePrint design and authoring, nothing needs to change...
The process for creating schemas, components and pages etc. remains the same, and the methodology of using multiple BluePrinted publications to, for example, localise and/or translate content remains valid and can be identical to that which you know and love.
Architecture wise - things can be a little different to the 'traditional' Tridion implementation...
Traditionally, one might publish different web site publications to different physical paths, or even different servers, and then serve them via multiple configured hosts.
With DD4T (or other techniques/frameworks that don't rely on physically published content), one could publish everything to a single broker database. There could also be just one web application that determines the correct publication ID to use (by the domain name or path used to access it, perhaps) and then uses that publication ID when querying the broker for the content to render.
Using this technique means that you can have an entire 'family' of publications served by a single host. For example, a customer I'm working with has 1 site that is localised and/or translated via BluePrinting for around 30 different markets. All of these publications are published to a single broker database and are served by a single DD4T based web application under different URLs (and paths in the case of multi-lingual sites).
NB: Note that I am specifically addressing just one possible configuration when using DD4T - As others have pointed out, there are many more! (The authoring processes still stay the same though)