I've only spent a little time with this, but here are some comments about CocoaFIBS. It is a great start, and I look forward to seeing it get even better.
The FIBS Login window needs a close button or a Cancel button (preferably both).
The metal look is gratuitous and distracting, I suggest using a normal window instead. The window doesn't need to be so small--after all, it quickly goes away.
The strings sound after logging in is also gratuitous and distracting. Sound effects in an application should always serve a purpose; this sound serves no purpose besides making noise.
Player list is very "wordy" with the repeating "Rating" and "Exp" for each player. May be unavoidable given the two line design (which is elegant, but it does require more scrolling.)
You might get rid of the parentheses around the player's current status, the font size difference is sufficient to distinguish the status text from the player's name. The same comment applies to chat messages; you don't need the [player] brackets around the player names since the font size/color already provides differentiation.
The two icons (status and client software) one above the other creates for a lot of visual noise, making it difficult to interpret either. The client icons are so tiny as to be almost useless.
The Sorting checkbox buttons are pretty wonky. I figured out what they do, but it really isn't the right design for this because it provides no information about what the primary sort order is, especially if more than two are checked.
Two popup menus (with choices [blank] / Name / Rating / Exp / etc...) might work better:
Sort first by: <popup>
then by: <popup>
I guess the Match Length control at the bottom of the lobby sets the match length to use when you invite someone, but it is not very natural or intuitive, at least to me. The Match Length control has nothing to do with sorting, but its location at the bottom of the drawer suggests that it is.
Your preferred match length should be a preference, and have menu commands to invite by the default, and possibly to invite "custom".
The board graphics are too cramped. Can't see the pip numbers, the artwork should have more room to "breath". Overall, the board is too small for most monitors, but I understand the issues and challenges of proper sizing all too well. In CocoaFIBS case, the problem is aggravated by having the player list in a drawer rather than a separate window (and speaking of which, having the Players appear as a choice under the Window menu is confusing.)
Double clicking to move is bad, bad, bad. It requires twice as many clicks as necessary; over the course of many games that is a considerable number, which directly translates to user fatigue. Moreover, double clicking requires more dexterity, which some computer users find difficult (or even impossible) to do.
I believe the GamesGrid design of single click to move first (left) dice, and control click to move second (right) dice is superior to what I did in MacFIBS; it is particularly nice with a multi-button mouse.
(Update: in 0.3 I see you offer an option for single click. Swapping the dice with Option-click is rather lame, though. It still requires an additional click that doesn't contribute to moving a checker.)
You must have an Undo Move menu command (toolbar buttons should never be the only way to perform a command). Assign it a command key equivalent so that players can set it for a multi-button mouse (I use the scroll wheel button to undo a move in GamesGrid).
The toolbar button for Undo Move should be in the default set.
Preferences window opens to default on Chat. If that's what you want, the Chat tab should be the first one. (Update: in 0.3, opens on Board. Make sure you set the TabView to the initial tab before closing the file in Interface Builder.)
The Prefs window is resizable, but it doesn't do the right thing. Generally speaking, preferences windows need not be user resizable (they may dynamically resize themselves, as the System Preferences window does.)
Using a slider to choose single vs double click is a clear misuse of the slider control.
The application icon doesn't appear correctly in my dock or the application switcher list, not sure why. Probably a bad .plist or something else in the app bundle.
The "Public Chat" menu command is not enabled, can only open it with the toolbar button.
Lose the tiny red text wherever it appears (login window, board, player drawer). Looks cheesy, is not very readable, and red has inherent connotations (read this! important!) that aren't applicable in the context you are using them.
More generally, the interface is too cramped when you use tiny size fonts, regardless of color.
Use Interface Builder to align your controls and other visual elements correctly. For example, the input text fields or popup menues in the Terminal and Public Chat windows should not touch the sides of the window, there should be space between them per Apple's UI design guidelines. Otherwise, people will think the application is written in (shudder) Java!