The shortcut to switch tabs is…
| TextMate: | Command + Option + Left/Right |
| Skype: | Command + Shift + Left/Right |
| Adium, Firefox: | Command + Left/Right |
| Colloquy, Firefox: | Command + Up/Down |
| Opera, Safari, Terminal: | Command + Shift + [/] |
Dear Santa, please, bring me Mk 41 next christmas, so I can fix this world.

That’s one of the small things that bothered me a lot about KDE as well (although i don’t know how that’s being done now in KDE 4). I’m glad i’m still a happy Gnome user.
This is quite irritating and Apple should fix this with their HIG, but it’s not all that bad. cmd-shift-left/right works also at least in Safari and Terminal. For the rest it can most likely be fixed via System Prefs / Keyboard.