Who you ask? Well the avid fans will know Jim to be the wonderfully talented voice actor for some of Valve's best characters including Father Grigori in Half-life 2 and the late Bill in Left 4 Dead.
The tweet simply says "Recording an old friend today." suggesting new lines for Bill will make their way into potential new Left 4 Dead DLC, or perhaps something to do with the Payday game collaboration with Overkill. Any truth in that or is it bull freaking horse shit? Who knows!