The Xcode source code editor allows you to choose the file from a list. It's two clicks instead of one (as it would be with tabbing), but it's better than nothing.
In addition, you can simply Alt-Tab through your open source code windows. This is not slower than tabbing, and has the same effect since the source code windows are usually placed exactly one in front of another.
First of all, you can use Textmate (which I believe has Xcode integration). Otherwise:
Window (Menu) -> Organizer (ctrl-command-o)
At the bottom of that window, if you don't have two panes, click the square to the right of the gear. Now drag code files of interest to the left, grey pane--a single click or arrow up/down will open the file in the editor pane.
If you do open a bunch of windows, as vog suggested, you'll need to command-~ through them--not alt-tab.
Not really, but one alternative is View > Show Favorites Bar and drag five or six frequently-used source files into it. Not as flexible as tabs but satisfies your request for "quickly navigate through a subset of files".
The traditional way is to use the detail view. Get the files you want in the Detail view by one of these means:
Put them all in the same group, then select the group
Enter a filter expression in the Search Bubble that narrows the items shown
Define a Smartgroup that includes just the files you want
Get a list of the files as a Find in Project result, then select that item in Find Results
Then you can use the Detail View as your list of interesting files and navigate through it quickly with the up and down arrows.
You may also try an Xcode plugin I've just released - it's called Code Pilot and solves a lot of issues of Xcode's navigation, making it more TextMate/Eclipse-like.