Acorn 2.5b1 is ready for testing on Flying Meat's latest builds page. The full release notes are also available, but I'll give a quick highlight as to what is new.
Finally, layer masks! This is a big new feature that people have been telling me for years that they just can't live without. To add a layer mask to a layer, control-click (or right click) on your layer in the layers list, and choose "Add Layer Mask". A layer mask works just like you'd expect from other image editors, and you can use all the regular bitmap tools and filters on as mask just like you'd expect.
And there are lots of little bug fixes, tweaks, and features such as matte color for web exporting, new AppleScript and JSTalk commands to add oval selections, you can now flip bezier curves horizontally or vertically, use the up and down arrow keys to change number values in more text fields, and more. Try it out, and of course we always want to hear your feedback: firstname.lastname@example.org