Just curious, I thought VML was still supported in IE9? Why the need to build an SVG solution?

Wikipedia confirmed it: "VML is still available in Internet Explorer 9".

Having said that, if you're starting to branch out into using SVG, any plans to make this a cross-platform (cross-browser) library for rendering CSS effects?

Oops, two separate topics for one post. Oh well, answer the question you like.

A few good reasons to use SVG rather than VML in IE9:

1) SVG can be used as a CSS background image, whereas VML requires inserting elements into the page. This makes PIE much less invasive in IE9 than in previous IEs, and much less error-prone (z-index issues go away, positioning never gets out of sync, etc.).
2) SVG is much more expressive than VML. There are many things that just cannot be done using VML (e.g. radial gradients, tiling of vector graphics, full rgba support, etc.)
3) SVG is hardware accelerated in IE9.

I have no plans to expand PIE to any other browsers, it seems they all (and even IE10 soon) have good enough support already.

Thanks for the explanation because I had also wondered why.

Ah! Never saw you answered this. Thanks from me as well for explaining.