I crossed through Uyuni, but they didn't stamp my passport at the Chilean border for some unknown reason. After realizing this, I attempted to get a stamp in Sucre AND Santa Cruz de la Sierra - neither would give me one without significant "contribution" (implied).
A couple months later at Copacabana, I ended up waiting until the Bolivian border crossing closed at 7pm and just walked to Peru where they happily gave me an entry visa in exchange for significantly less money than the Bolivian visa. <_<