There were rumors of a Samsung tie-up too last year, but nothing came of that: http://www.koreatimes.co.kr/www/news/tech/2015/02/133_173217.html In general, it's not impossible for them to source similar cells, but suspect it'll be mainly to fill gaps while the Gigafactory is still building. Unless LG Chem also invests in the Gigafactory, Panasonic will still be the primary player.
I think they are using Samsung cells: http://www.koreatimes.co.kr/www/news/tech/2015/02/133_173217.html They just need even more. It's funny, a bear argument is that "LG is going to supply all the other automakers once the demand for EVs materializes". But Tesla can guarantee much larger volumes for LG, so why wouldn't LG sell to the biggest player? This squeezes the cell supply for everyone else even more.
Barron's Koreas LG Chem To Tie-Up With Tesla As Volkswagen Scandal Fuels Electric Cars - Asia Stocks to Watch - Barrons.com - - - Updated - - - Bingo
OK...but Barron's has a reputation to uphold...I'm feeling a bit more comfortable with the article...
According to this page, the LG Chem batteries are for the Roadster upgrade. Which makes some sense, Panasonic is probably maxxed out supplying Model S and X batteries and they want to get the Roadster upgrade going. http://www.teslarati.com/tesla-partners-lg-chem-roadster-battery-upgrade/
Don't see what the big deal is. Tesla has always said they may source batteries from other suppliers. This is a smaller deal for Roadster packs.