Rounding corners is easy. With CSS properties like border-top-left-radius, you can even control each corner differently. This can apply to any block, not just divs.
Floating Blocks is also easy.
Pardon my floataceous behavior. And also notice that you can incur a "uneven" look when the content of one block makes it longer than the other.
Use the clear property to break out of the float. Remove the clear from this block's style class and see what happens.