Those who aren't comfortable in all keys can try this:
Find a small church that really, really needs a keyboardist. This type of church needs to have one of those old mothers who always strikes up a throw back right after devotion or something... or a praise team that sings in any and every key... or tone-deaf soloists. Now, the church may not pay much (if at all), but you'll be thrown into a situation where you're going to have to follow them and play in whatever key they start singing in.
First: this will give you the opportunity to test out your skills in a smaller, less demanding environment. Trying to test out your skills in playing in all 12 keys at your local mega church isn't the best idea. You'll be too concerned about the crowd and the pressure to really focus.
Second: you won't have to worry too much about making mistakes. Why? Because sometimes these churches need a keyboardist so bad, they aren't going to be concerned if you hit a couple wrong chords here or there. Even if they don't need a keyboardist bad, sometimes they just don't care. As long as they're jammin' to some music, they'll be satisfied.
Third: the more you play there and hash out your weaknesses, the more your confidence will build. And, sometimes these churches are more liberal, so you can jam and jam and jam all day long.
I do this now. The church I play at most Sundays has an average attendance of 15-25.. maybe 30. They are LIBERAL and they've got a kickin C-3 (it's a little beat up and missing a key but it still jams). So, I can play most whatever as long as they like it (and they usually like most of it). They sing everything from old southern traditional jams to stuff by Israel. The thing is that their praise team and choir aren't any good at keeping notes or keys, so 98% of the time, they start the song and I follow them. That means they could end up in F#, A, B or E at any moment. It has incredibly helped me become more comfortable in all keys.
