There were actually a couple of tweaks released on Cydia a week or two ago that do just this.
Transparent Clock Background iOS4
Transparent Slider Background iOS4
I don't typically use the stock clock or slider, but I grabbed them anyway just to see how they worked. They work great, frankly. Yes, you need WinterBoard to use them, but if you don't have a problem with using it, then this should be what you're looking for.