Very Limited Soloing in DDO Victor "Sporkfire" Wachter responded to a forum member's question about whether solo play will be a viable means of progression in DDO with the following:

It remains primarily group content. You can solo through a good chuck of levels 1-3, but not the entire game. Ultimately, it serves the D&D rules best and is the type of content we can do the most interesting gameplay with. Characters in D&D are designed to be best played with multiple people filling different roles, and we keep that same philosophy in the game.

See the entire thread [HERE].