Grognardia gives a nice summary of the Ages of D&D here. While you can argue some of the exact dates his summary dovetails nicely with my memories of how the D&D changed over the years. One of the comments he ends the summary with is about D&D 3.0 and it successors.
The advent of the Open Gaming License associated with the 3.0 edition had several beneficial impact for fans of the original games. Many of these benefits took several years to realized but all of them contributed to the revival of Old School gaming.
#1) This point is independent of the OGL, D&D 3.0 succeeded in returning many people into the hobby. The impact on Old School gaming was that not only the number of gamers were increased but many were involved again who played in the 70s and 80s.
#2) The pool of potential old school gamers was not only increased in quantity but in diversity. A wider range of resources, skills, and talents were avaliable in the world of gaming.
#3) The OGL lowered the barrier of entry into the market but removing the requirement that you had to have your own RPG. Now people can focused on settings, adventures, races, etc.
The combination of the previous two paragraphs lead to the formation of many third party companies. Necromancer Games, Goodman, Games, Troll Lord Games, Green Ronin, etc. Several were founded explictly to cater to older gamers notably Necromancer Games. While others later started product lines that appealed to older gamers and sold well to everyone. (Goodman Game's Dungeon Crawl Classics).
Several author of old school products, including myself, got their start writing for one or more of these companies.
The big problems of using a out of print game is twofold. First there is copyright. In the US rule mechanics can't be copyright. This is great for something like Tunnels & Trolls or any other RPG that is similar but not the same as D&D. However there is the problem of trade dress. In the US the law frowns on competitors making items that pretend they are an offical part of another's product line.
However the OGL made nearly all of the trade dress that needed for a older D&D clone avaliable provided that your rules follow the OGL.
Castles and Crusades was the first major attempt at this and became the backbone of Troll Lordl's Product line. Dissatification with Castles and Crusades lead to OSRIC, Labyrinth Lord, and other retro clones.
Finally the continued rise of the internet made a PDF market viable. Not only further lowering the barriers of entry, PDFs can make avaliable older material that publishers feel are too unencomical to print. Within a handful of years nearly all of the material for the older editions were made avaliable.
The retro-clones are still useful even with the older rules avaliable as often they are easier to learn from. They also serve useful to publishers as the definition of what the boundaries are in reusing older content.
2008 saw a continuing expansion of NEW material for the older edition. Even a bit of controversy with the Carcosa supplement for original D&D. It been building up but somewhere in 2007 to 2008 everything started to click.
I see continual slow but steady growth in the market for material for older editions. I do feel that if we want to expand out base we are going to have to come up with RPG material that not found in any other market. Something that takes advantage of the strength of older edition but yet is completely new.