Connection of the exposed paddle really depends on the IC, so it is extremely
important to read the datasheet before making a decision. Typically, it is
recommended that the exposed paddle be soldered to the PC board ground plane
for best thermal transfer (see table 5, page 8 of AD8139 Datasheet Rev. B
10/2007). In some cases the exposed paddle may be soldered to the most negative
power plane, in such a case this would most likely be specified.
With the AD8139, connecting the exposed paddle to ground is recommended.
However, from table 5, it appears the exposed paddle is not internally
connected, so it can be connected to any power plane (ground or power) which
will act as a heat sink.