Amazon is offering $50 of in-store credit with Vita purchases between now and June 8. The offer's good for either the Wi-Fi model or the 3G bundle, and the $50 doesn't have to go toward Vita games – although Soul Sacrifice is going for $30 at the mo.
A word of caution: as Tiny Cartridge notes, the timing's a wee bit suspect. One of the most common entries on bingo cards for Sony's E3 conference must be "Vita price cut" - possibly only beaten by "The Last Guardian lol." After all, Sony's already slashed the Vita's price in Japan, and the sales forecast for the fiscal year wasn't great. Just worth bearing in mind.
Update: Variety is the spice of life - Best Buy and Target are running similar offers too. Walmart stores are apparently in on the act as well.