In Emacs 23.2 and higher, the dired-x.el module is available, and it gives you access to a command that does exactly what you want. After you load it (just (load "dired-x"), normally), you’ll be able to invoke the function dired-do-find-marked-files. Here’s its built-in documentation:
(dired-do-find-marked-files &optional NOSELECT)
Find all marked files displaying all of them simultaneously.
With optional NOSELECT just find files but do not select them.
The current window is split across all files marked, as evenly as possible.
Remaining lines go to bottom-most window. The number of files that can be
displayed this way is restricted by the height of the current window and
`window-min-height'.
To keep dired buffer displayed, type C-x 2 first.
To display just marked files, type C-x 1 first.
So after dired-x is loaded, you can just use M-x dired-do-find-marked-files RET and you’ll get exactly what your question asks for: all marked files will be visited as though you’d run dired-find-file on all of them.