There isn't a MCNostalgia method of just flicking between versions, mainly because of the difficulty that would cause with the different forge and mod versions.
There are packs on different versions, though (the beta pack is currently 1.4.2, DW and Mindcrack are 1.4.6, I think), and you can go into each and pick from previous versions of the packs (i.e. from v1 to v4 on the DW20 pack).
If you startup a different pack, it leaves your other files undisturbed, and makes a new "instance" for that pack (so you can flick between packs easily).
So, basically, you can access any version of MC that has been in the launcher so far, but not anything else.
