I seem to remember it being possible years back, but can't quantify that exactly.

The page which lists newly available (ie deleted and purged) account names has existed since at least mid-2007:

http://web.archive.org/web/*/http://www.livejournal.com/misc/expunged_list.bml

It looks like it wasn't possible to reuse a username in December 2004:

http://web.archive.org/web/20041216132254/http://www.livejournal.com/support/faqbrowse.bml?faqid=127

But has been possible since at least March 2006:

http://web.archive.org/web/20060318105519/http://www.livejournal.com/support/faqbrowse.bml?faqid=127