The problem with just doing the simple s/-/+/g is that vercmp treats + identically to . and therefore the resulting version number is 'newer' than actual point releases:

"warning: zeitgeist: local (1.0+1+g1bcc8585-2) is newer than extra (1.0.1-1)"

Adding some letters would avoid this problem (unless you like bumping epoch every time this happens). My favourite is:

pkgver() { cd $pkgname; git describe --tags | sed 's/-/.r/; s/-/./g'; }

This would result in 1.0.r1.g1bcc8585-2, which correctly compares as 'older' than 1.0.1-1 (and I like the dots, but it works equally well with +'s).