Is there a more recent bibliography style file (.bst) for PNAS?

I'm not aware of a BibTeX style file for PNAS, but the Bibulous project does provide an easy way of customizing styles. For the style suggestions linked to by the OP, it took me only a few minutes to put together a complete style template to follow PNAS' requirements. Using the following main.bib database file

@ARTICLE{Neuhaus,
  author = {Jean-Marc Neuhaus and Liliane Sitcher and Meins, Jr, Frederick and Thomas Boller},
  year = {1991},
  title = {A short C-terminal sequence is necessary and sufficient for the targeting of chitinases to the plant vacuole},
  journal = {Proc Natl Acad Sci USA},
  volume = {88},
  number = {22},
  pages = {10362-10366}
}

@INCOLLECTION{Hill,
  author = {Adrian V. S. Hill},
  year = {1991},
  title = {HLA associations with malaria in Africa: some implications for MHC evolution},
  booktitle = {Molecular Evolution of the Major Histocompatibility Complex},
  editor = {Jan Klein and Dagmar Klein},
  publisher = {Springer},
  address = {Heidelberg},
  pages = {403-420}
}

and the style template file main.bst (the lines below show the complete file)

TEMPLATES:
article = <au> (<year>) <title>. \textit{<journal>} <volume>(<number>): [<startpage>--<endpage>|<startpage>|<eid>|].[ <note>]
incollection = <au> (<year>) <title>. \textit{<booktitle>}[, vol.~<volume>, ][, <edition_ordinal>~ed.][, <null.if_singular(editorlist, edmsg1, edmsg2)>~<ed>][, <series>][, Chap.~<chapter>] (<publisher>, <address>)[, pp~<startpage>--<endpage>|p~<startpage>|<eid>|].[ <note>]

SPECIAL-TEMPLATES:
authorlist = <author.to_namelist()>
editorlist = <editor.to_namelist()>
authorname.n = [<authorlist.n.prefix> ]<authorlist.n.last>[ <authorlist.n.first.initial()>][<authorlist.n.middle.initial().compress()>][, <authorlist.n.suffix>]
au = <authorname.0>, ..., <authorname.9>
editorname.n = [<editorlist.n.prefix> ]<editorlist.n.last>[ <editorlist.n.first.initial()>][<editorlist.n.middle.initial().compress()>][, <editorlist.n.suffix>]
ed = <editorname.0>, ..., <editorname.9>
null = {}

OPTIONS:
edmsg1 = ed
edmsg2 = eds

compiling the main.tex file

\documentclass{article}
\usepackage[paper=letterpaper, text={6.5in,9in},centering]{geometry}

\makeatletter %
   \renewcommand{\@biblabel}[1]{#1.}
\makeatother

\begin{document}
\nocite{Neuhaus,Hill}
\bibliography{temp}
\bibliographystyle{temp}
\end{document}

produces the following formatted result:

Formatted reference list

This provides templates for only journal articles and articles/chapters in books, but the PNAS website provides guidelines for only these two. Templates for other entry types are easily derived from the two shown here. (For example, a book entry type template can be defined by adding another line

book = <au> (<year>) <title>. ...

in the lines below TEMPLATE: in the style template file.)


Also check out https://github.com/jburon/pnas2011.bst for PNAS bibtex style updated to match the current fashion in reference layout as of 4 September 2011.