Borderlands and Demon's Souls were two of 2009's biggest hits, measured by sales and/or popular acclaim. So there's a nice symmetry in seeing Borderlands 2 paying homage to Dark Souls. Spoilers ahead!
The Easter egg can be found in the Caustic Caverns. The NPC's name is Solitaire, which might reference Solaire of Astoria, a character in Dark Souls