It would be a nice addition to have round cornered rectagles... unless this can already be done..

It can already be done!

Ok, I probably should have mentioned how to do it.

Use the 'Edit Vertices pointer' (the white arrow) and select a rectangle. You will see 3 small control points on both the upper left and lower right corners. Choose a corner and drag the two outer control points away from the corner to create the rounding.

I think there is some room for us to improve the rounded rectangles here. There can be a bit of uneveness in the line in if the coners are not symetrical. (this unevenness does come through in SVG since SVG renders the rounding)

Ok... corners a little bit rough though... Would it not be better to have a seperate Rectangle that comes with rounded corners?.....

...and even a four pointed triangle and a cornered circle?