Anyone that currently uses Yahoo Messenger knows it already has a "push to talk" capability that lets you talk PC-to-PC to someone on your buddy list.
Well, at SuperComm 2004, Yahoo announced it would be supporting "click to call" over the PSTN for making PC-to-phone calls. Two possible PSTN providers include SBC Communications and Verizon Communications, which utilize Sylantro's VoIP infrastructure that Yahoo also uses.
With Yahoo's strong presence in the IM space, seeing them getting into VoIP with PSTN access capabilities will surely cause some heads to turn, especially Microsoft (Messenger) and AOL (Instant Messenger).
People love their "free" instant messages and "free" push-to-talk capabilities with their IM programs, so whether or not a traditionally "free" IM client can finally make a profit by offering PC-to-Phone calling remains to be seen.