Per this article published while the GPL 3.0 was drafted (and mentioned by apsilers):
We have made two changes that recognize and facilitate distribution of covered works in object code form using BitTorrent or similar peer-to-peer methods. First, under new subsection 6e, if a licensee conveys such a work using peer-to-peer transmission, that licensee is in compliance with section 6 so long as the licensee knows, and informs other peers where, the object code and its Corresponding Source are publicly available at no charge under subsection 6d. The Corresponding Source therefore need not be provided through the peer-to-peer system that was used for providing the binary. Second, we have revised section 9 to make clear that ancillary propagation of a covered work that occurs as part of the process of peer-to-peer file transmission does not require acceptance, just as mere receipt and execution of the Program does not require acceptance. Such ancillary propagation is permitted without limitation or further obligation.
Based on this, section 6e of the GPL 3.0 is now quite explicit about peer-to-peer transmissions:
e) Convey the object code using peer-to-peer transmission, provided you inform other peers where the object code and Corresponding Source of the work are being offered to the general public at no charge under subsection 6d.
But then what about the GPL 2.0 especially in the light of the comment you made?
First, I think this is rather artificial to treat a peer-to-peer redistribution any differently than a regular distribution by download.
In particular, if I host download at a hoster, I would not consider the hoster to be responsible for the redistribution, nor any of the many internet and network segments providers where the bits did transit.
As an analogy, I would not consider the peers and seeders participating in a p2p transmission as responsible for the redistribution of these bits proper. They are in both case just instrumental IMHO.
So who would be responsible for the redistribution and eventually responsible to provide eventually the corresponding sources? Whoever initially created the torrent, e.g. the first seed would the redistributing party IMHO and should therefore ensure that the corresponding source code is available in one of the many ways that the GPL 2.0 supports, such as at the end of Section 3 of the GPl 2.0:
If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.
... which could be in the form of another torrent for source code, or any suitable mechanism.
And the torrent should also contain instructions and pointers on how to obtain the corresponding sources either directly in the main ISO or as an additional README.
And as you pointed out, most ISOs contain information about how to get the source code and therefore regardless of how they were provisioned they would be compliant be it through a p2p or http download.
