[Geodar]

I think you can't go far wrong in a smart suit, shirt & tie, you can easily take the jacket & tie off. Ladies in a smart trouser suit and blouse, again the jacket can be removed.
As it is a wedding and the guests will be dressed up to the nines (at most weddings) then it is important you look the part, as said earlier you never know who you impress and may get the next engagement/wedding shoot out of it, of course a lot depends on your manner and photo results as well.
If you are doing and engagement then dress down slightly, smart pants & shirt. A kids party (depends on location), dress casual.
In my experience people respond more to someone in a suit & tie (it seems to gain more respect), they see the suit and think he/she is a professional with an air of authority, which is what you want to ensure you get the shots you are being paid to get.
Geoff