It occurs to me that the ability to use rituals is going to be mandatory for all characters at epic levels, in order to travel to the places where a lot of epic adventuring is apparently meant to take place: the planes.
I can't see this working unless every single character class has access to a generic planar travel ritual. It would be cumbersome and restrictive to play options if only certain classes got access to these rituals - I can't imagine that a party of a Warlord, Fighter, Rogue and Ranger is intended to be unviable for epic-level play. So I conclude that at least one of those classes needs access to a planar travel ritual. And once one of them can do it, I see no reason to deny it to the rest.
We know that cleric and wizard get the ability to use rituals at first level (or rather, they did at the time the characters for DDXP were created - we know at least one rule has changed since then), but how do other classes get access to rituals?
I was thinking at first it may be a feat, but if my guess about all classes getting access to planar travel rituals is correct, then it shouldn't be possible for an inexperienced troupe to shoot themselves in the foot by no-one thinking to take the feat.
So, my current theory is that all classes get access to rituals, but not all at the same level. I furthermore guess that there will be rituals that any class can use, and others that can only be used by, say, classes linked to a specific power source.