There are two video effects that you can use... under the Transform bin -- they are called Vertical Flip and Horizontal flip. i use premiere pro 1.5 so i think it's the same but i'm not sure.
so, you mean to tell me that you already edited everything and NOW you want to flip your image? -- i don't believe there is any way to do it all at once. i could be wrong.
If all the clips are on the timeline, create a new sequence, then drag your original sequence (Sequence 1, in the project window) into the new sequence. Then apply whatever filter to that 1 clip.
I don't think you can apply a filter to clips that aren't in a timeline. Such as flipping all your clips before you start editing. You could stand on your head and edit.