Hi Joe,
Yeah, for now you just have to make a circle raster-layer to mask the canvas. Super early on before the first release there was a plan to support cylindrical canvas shapes but so much has been built onto everything since then that I'm thinking it's just going to stay the way it is with the canvas being a rectangular shape. After this next release of v1.61b soon the focus will be on the new content library system where users can add content from an online public library where users can share shapes and designs as paths-layers and raster-layers. There will be a base library of shapes like squares, rectangles, circles, stars, arrows, etc... which will make it easy to add these things to a project for whatever purpose, including masking off a canvas for a round project.
To put text on a curve you would create a paths-layer, draw out the curve you want - I prefer to use the curve as the centerline for the text, but you can draw a curve and have it be for the bottom or top of the text. Then set the paths-layer with the curve you want to use as the layer beneath the text-layer you want to conform to it by re-ordering the layers within the canvas layers list. While the text-layer is selected you then use the Text Along Paths function, under Layer Actions on the left side of the screen.
A pretty thorough explanation of how to use the Text/Raster Along Path(s) function can be seen in the PixelCNC Tutorial #03 video here:
(EDIT: The Along Paths function explanation begins at the 13:00 minute mark)
Let me know if you have any other questions about the Along Paths function :)
- Charlie