The ability to stream content from your PC already existed on Roku players using Roku channels like Plex, PlayOn, Gabby, etc. PlayLater is available (for purchase) and functions like a DVR. I have been using PlayOn for a while and it works rather well.
Miracast, in the PC world, can be used to wirelessly add another monitor to one's PC. This is different than Plex (which I love), PlayOn, and so forth, in that one does not simply broadcast media content, but actually uses the Miracast device as a second monitor, thus enabling one to compose documents, surf the web, and run other apps.
Question, Sean: When is the ability coming? I looked in the Roku channels store, and even through various private channels sites, and I did not see an available channel. And "later this year" has definitely arrived. I look forward to Miracast, on my Roku 3, but aside from this forum, I have not seen it advertised as an impending feature. Roku's site itself is mute on the subject. Aside from rumor, on the Roku forums, I have not seen an official announcement about Miracast availability this year for the Roku.