LEGO and NI have added something to the support pages - a patch that allows you to add in new NXT-G blocks seamlessly into the environment. Take a look here:
Note that with the ability to add blocks in, you really need new blocks to add... so they have provided those as well: miniblocks (more streamlined versions of the "stock" blocks... more on that in another post) as well as the legacy blocks that work with the older RCX system sensors and motors (these were already released in the educational version of NXT-G, but now you can add them to the retail version, free of charge).
Addendum: Ah, I misunderstood - check out a just-now-appearing post under 13 Nov on this blog, for more details on the memory-saving abilities of the miniblocks, etc. I wrote it then, but didn't want to post it until the release was availible... and now it shows up there. Here's a link back:
Introducing the Miniblocks