Seems like a lot of people are using some round-about methods of finding the center of objects.
Things to bear in mind:
1. If you have snapping turned on, guides snap to the center of the object on screen. If you have a layer filled with pixels or a background layer, you will get the center of the document. If you have an object, you will snap to the center of the object (no need to pull up the transform bounds). That little center reticle is actually for determining the origin of the transformation. If you slide that little reticle around, you will change where the trasformation occurs from.
2. When you create a new guide from the menu you can manually type in any units you want. The method of typing in 50% to get the center of the document was already mentioned. I built this into an action for myself and set it to an f-key right next to my gaussian blur function
3. If you need to center an object, you can snap it to guides by moving it until the center snaps into position. One nifty thing to remember about moving objects from document to document (this is often when I want something centered) is to hold the shift key while you drag the layer from one to the other, this will cause what you are dragging to appear in the center of your document rather than arbitrarily placing it based on your mouse position. Anyone familiar with Illustrator will also recognize the layer alignment options that have started appearing in Photoshop in the last couple of versions. If you click on your background layer or a layer filled with pixels, then link your object layer to that layer and click the align center horiz and align center vert (you even get buttons to do thi if you are using the move tool) it will align the layer to the center of the document.
4. Guides can always be repositioned to another area if you want to start snapping objects to another location after you've built them. Just move the guides and then slide the object until it snaps to the
new origin point.
Hope that was helpful in some way. Like the sayin goes "There are a million ways to do a million things in Photoshop, and they're all correct."
Carry on