The official site says that "new mini-sodes" start this Tuesday June 24th at 10 PM. Unfortunately it doesn't explain what the heck a "mini-sode" is, nor does it say anything about when the new season starts.
I programmed my DVR to start taping them as of Tuesday June 24. They're six minutes long and are every Tuesday night at 10 for the time being. Denis Leary talked about this on "The Daily Show" a month or so ago as a way to fill the gap for fans. The 5th season was pushed back because of last winter's writer's strike. We're not going to see Season 5 until next spring at the earliest.
A "mini-sode" is a term that, as I understand it, is owned by Sony. They originated it by taking half-hour and hour-long episodes of classic shows (usually sitcoms, like Diff'rent Strokes) and mega-editing them down to just the super-basics. The result is a 3-to-6 minutes long...thing.
Younger people seem to LIKE these, for reasons which I frankly cannot fathom. Sony includes two of them as regular (and the only) "extras" on most of their recent DVD sets, and there is even a "Minisode" network on MySpace: MySpace.com
Now, at other studios, many shows get "webisodes" or "mobisodes". These are essentially NEW material, made in short form, to fill in the gaps not shown in a proper episode of a story. Sometimes they are actually made from existing deleted scenes, but more often they are made brand-new, just to present on the internet (webisodes) or transmitted to mobile phones (mobisodes; almost always available later on the web).
Lost had an excellent, but eclectic group of mobisodes called "Missing Pieces" that were some deleted scenes, some new material, that filled in a few blanks (or just gave you a chuckle or two) but were not necessary to the overall storyline. Lost - Missing Pieces Home - ABC.com
Battlestar Galactica had a series of webisodes after Season 2 called "The Resistance", which most fans felt were VITAL to understanding what came next at the beginning of Season 3 (I sorta agree, but not entirely). Webisode 1 | Videos | SCI FI Video
I'm expecting that the "Rescue Me" mini-sodes are the same as webisodes or mobisodes, just using Sony's proprietary term (I strongly believe it's proprietary to them). Something to tide fans over until they release the next season. I'm not saying I'm not a fan of RM or anything; I simply haven't started watching the show yet (I have at least two seasons of the show languishing in my VERY LARGE "need to watch this" pile!), so keep in mind that I haven't explored exactly what's going on with these minisodes. Therefore, take what I say with a grain of salt.