I wrote up an ATOM 1.0 renderer for pyBlosxom tonight based on Blake Winton / Will Guaraldi's RSS2Renderer plugin.

AtomRenderer renders a valid ATOM 1.0 feed without the need for ATOM flavour templates. You can access mine at http://www.timfanelli.com/atom. You can validate my output here.

You can download AtomRenderer here

Update: (Nov 1) Just got an email from Will Guaraldi letting me know that pyBlosxom 1.3 will have a native ATOM flavour for generating feeds -- so this plugin is really only useful for pyBlosxom 1.2 and earlier :).