Stig got me curious, so I thought I'd see what was up.
Sounds interesting. I haven't ever thought about it - I mean really, why would you want to convert a spline to arcs, anyway? - but it does sound like you might be able to pull it off by stepping through the spline and finding the radius at each point by using the derivative. I'm not sure how splines are set up - let me know what you find out. I don't have time to explore it right now. I have noticed that the params go on splines like they do on lines, not like on polylines, so you can't do something simple like draw arcs between the params. Unless, of course, it turns out that that's what a spline IS, a series of 1-unit-long arcs...
It's always possible (even likely) that the spline is implemented as a spline at a REALLY low level, and it isn't made up a series of arcs at all (the way a spiral curve is in LDD - or is a spiral a series of splines? yet another thing to explore...). In that case, all you can do is approximate the spiral with curves by picking points at regular intervals, and declaring those the endpoints of your arc segments. You can use the derivative to figure the tangent bearing at each point, and then try drawing a curve using a tan-tan fit. I don't know if this will work - it might be a null solution set. In that case, you would have to have tiny angle points between each arc. The easiest way to do this would be to use the derivative to get the radius at each endpoint, and use the average as the radius of the arc segment. Of course, if using the derivative to find a radius ever breaks down, this is probably how you'll find out.
Here, the only problem is that having arcs of equal length is inefficient. You need more arcs where the spline is changing the most. It sounds like you actually also might want a thirdDerivative function. Then you could make the length of the arc segments inversely proportional to the average magnitude of the third derivative over that segment.
Note that I'm happy to sit here and spout things, and let you do the actual coding...
Enjoy!