@JECE, actually, yes it was. The local Romanian speaking population was deported, to places such as Siberia or Kazakhstan, and Russians were moved in.
The area was inhabited since antiquity by Thracian* (not Slavic) people, even the Romans at one point had a presence there. *As you may or may not know, the ancestors of the Romanian people are the Dacians (Thracian people) and the Romans. Even the names of cities and such in transnistria bear 'Russianized' Romanian names. Yes, the population there being Russian is ENTIRELY due to population transfers.
http://en.wikipedia.org/wiki/File:BASARABIA_ETNIC.png
http://en.wikipedia.org/wiki/File:Harta_etnica_a_Republicii_Moldova_-_2004.jpg
Also, annexing, as said Jamie, is of 0 strategic importance to Russia, ruining its ties with it's neighbour Romania (which wants to regain Moldova) would be a terrible move (they're already not the best anyways).
Imo, let those fucktards leave, one less problem for Moldova.