I think you're trying too hard. There are existing protocols (S/MIME, PGP, Jabber, etc.) for transferring messages in a secure manner. I'd pick the one that fits your scenario and wrap it up in a shiny app.
AFAIK, there are several IM apps that could support encryption, but I don't think they are actually doing so (and if they are, they aren't advertising it). As it is, end-end secure communications is not (I think) on anyone's feature bullet-list.
eh, there exist plenty of things that are nice and shiny.
I can take pidgin and OTR and with about three minutes worth of "You should click here", have it set up easily enough that even a complete non techy can use it.
The problem is 1) It requires installation and 2) People don't know it exists. (Strangely enough, once it's installed i've never had anybody move back, mostly as pidgin is a fair amount nicer to use then MSN)
You may be right, however one advantage of this method is that any messages and files are seen as exactly the same: a secure truecrypt encrypted file transfer.
Truecrypt is a container for storing a collection of files. Unless you're interested in the deniability aspects of Truecrypt where an alternate passphrase yields decoy data, off-the-shelf SSL with sufficiently large keys is more than adequate to secure the transport layer. I suspect your biggest challenge there would revolve around key authentication to prevent MITM attacks.
In the context of this discussion, you are talking about creating a crypto solution that prevents the NSA from sniffing your customers. Not a casual packet sniffer, the NSA. This is not the time to be running off and implementing your own crypto!
AFAIK, there are several IM apps that could support encryption, but I don't think they are actually doing so (and if they are, they aren't advertising it). As it is, end-end secure communications is not (I think) on anyone's feature bullet-list.