I've been using libmemcached v0.39 for a really long time, despite it having progressed up to version 0.51, and it got me thinking why it is that I'm using that. I simply forgot.
Doing some quick googling, I "remembered" my reason: the latest stable version of the PECL memcached extension is 1.0.2 (released 2010-05-03). This extension for PHP seems to only have been directly tested against libmemcached v0.39 as can be seen via it's Package Information page.
Therefore despite there being a fairly recent release of the PECL extension (2.0.0b2; only a month or so old) which seems to work on libmemcached version 0.49 and below, I would be taking a gamble on one of two fronts:
Based on those two options, neither of which I like, I think sticking with libmemcached 0.39 and PECL memcached 1.0.2 seems like the smart move.