In terms of GChat and Skype? Yes. You could even try over 2G but it wouldn't be pretty (not like it is already).

In terms of from a 3G phone to 3G phone (mostly seen in Europe and Japan, not in the USA) then no. Note: This does not mean you cannot use GTalk or Skype to talk to someone else who is using Skype on another phone. These are two seperate things.