Update: More details have emerged. According to SamMobile, the "Galaxy S III will be the company’s first handset to be updated to Android 4.1 Jelly Bean, followed by the Galaxy S II and Galaxy Note". The flagship Galaxy S III will reportedly receive an update by the end of the Q3 with the other two Galaxy devices receiving one in Q4.
Samsung has been surprisingly mum about the issue of Jelly Bean updates unlike HTC and ASUS, both of which have made official statements with regards to devices getting the recent Android 4.1 update. That doesn't mean that they are resting on their laurels - according to SamMobile, insiders have said that the Korean company has already started testing Jelly Bean on their popular flagship smartphone, the Samsung Galaxy S III. According to Yonhap News Agency, a Samsung senior executive has remarked that the global sales of Galaxy S III devices have surpassed "the 10 million mark less than two months after its official release", which also makes it all the more imperative that the handset receives the update as soon as possible.
Allegedly, an update for the older Samsung Galaxy S II is also being tested as well, but due to its older hardware, it is less certain that the device will hold up well with the new OS.
GSMArena - "The company will reportedly place more focus on updating its high-end devices, so the Galaxy Note and Galaxy Tab 7.7 (which use pretty much the same internals as the S II) also have a chance of getting an update."
Samsung is reportedly close to finishing up the update on the Galaxy S III. While there's no official word on the issue yet, GSMArena has mentioned that an August or September time frame would be realistic.