V.W.
Wow... my husband was the first to pick up on this at our house too. He made sure to explain it as a respect issue... not just rudeness. If two adults are having a conversation, then the child needs to wait for the adults to finish (or at least pause) before saying "excuse me" and getting permission to interrupt-- barring an explosion/flames or a situation involving spurting blood.
It is disrespectful for the child to behave as if their desire to talk is more important than the adults' ongoing conversation.
It is something that must be enforced constantly at first... over time as you can see the child approach, pause and think about what to do, and then wait.. but eventually lose patience and interrupt anyway.. you can be a little less stern about it. :)
Maybe talk to your wife about the disrespect angle... she may not realize that she interrupts often too, but whether she does or not, she will be able to see your daughter being disrespectful of you/her when SHE interrupts.
I wouldn't say anything to your wife about HER interrupting habit. At least not during the conversation about your daughter. That will be perceived as an attack (and denigrating her mothering skills as to what she is teaching your daughter) and you will get LESS cooperation about your daughter's issue. You might get lucky, though, and as she becomes more aware of your daughter's interrupting, she may curtail her own as well...
Good luck.
:)