To the Player
My advice to the "unreliable player" is multi-fold...
First, and foremost: communicate with your group and GM about it.
Second, make certain that a copy of your character remains with the group - either a reliable player, an online copy, or a photocopy.
Third, don't play a character integral to any major plot developments. Be the best supporting actor, but not a lead.
Fourth, pick someone to run your character for you when you can't. This may or may not be the guy who brings your sheet to game, too...
In a particular Hero System campaign of a friend of mine, one player bought a "teleport, no conscious control, once per session" to explain his sudden disappearances from the campaign... and his reappearances. It meant that the GM had a way to bring him into or out of the session, as needed.
Advice to the group
Now, some genres are better than others for this, and some play styles are better for this as well, but see if the group is willing to play more episodically than contiguously. Likewise, more mission-based games tend to be better than cohesive self-motivated party type games.
A Trek-style or SG-1 style game is better for missing player type groups than is a Farscape, Starhunter, or Firefly type game, for example. Trek and SG-1, a missing player can be replaced with an NPC or guest player much more easily than where there is a very limited cast, and anyone missing is both noticed and unreplaced. For Fantasy, being minor landholders doing missions for your liege (ala Pendragon) is better than trying to emulate a long hero's journey.