wouldn't it be better to include a posibillity to create non-linear and non-circular gradients, that is, gradients that start somewhere inside the object and end at its borders. not just because of drop shadow, but because it's a nice thing to do with vector graphics... or is it a svg limitation?
I'd really love to be able to do that. Circular gradients are sometimes very restricting. However, non elliptical/circular/linear gradients aren't part of the spec (see http://www.w3.org/TR/SVG/pservers.html#RadialGradients). There might be a way to get the same sort of effect, but I'm not familiar enough with the spec to be able to tell you that.
From what I can tell the type of gradient you are talking about isn't available in most vector graphics editors, although I agree that it would be a nice option to have. One exception I know of is Macromedia Fireworks MX which refers to it as a "contour gradient". However, if this isn't part of the SVG spec is seems like it would probably be difficult to add this type of gradient to Inkscape.
-Michael