Sort of...
I done something some what similar to this.
I started a weekly game last spring with a group of players here, and another friend of ours was suppsoed to join when his contract finished in another province.
To keep him up-to-date with the campaign, I ran an PbeM solo campaign with his character while the rest of the group did their regular thing. He'd always give plenty of advance notice if he was coming home for a weekend or two, and during those times the PbeM storyline and the face-to-face storyline would meet up and all would play together (as an aside, we've now got a webcam/mic thing set up, so he's joining our weekly sessions via teleconferencing).
As well, I often have certain encounters I want to progress through during a session, but for one reason or another, we don't always get through them all. If the remaining encounters are just exposition or role-playing, I'll usually wrap them up in an email or two rather than spending the first bit of the next session rehashing stuff.
As for how to accomplish this on a regular basis, follow the general modus operandi of PbeM games (I think Sean Reynold's has a PbeM FAQ maintained on his website, and there's a site with an address akin to
http://www.pbem.org with more info) for the email portion, and run the face-to-face sessions as normal. Since you won't have to worry about different pacing or groups like I did, it shouldn't be too difficult to switch gears as long as your players check email regularly. I suggest following Sayburr's advice of ending the email 'session' if a combat or near-combat encounter arises.