Xml & Xslt

Advert test
How do search engines treat XML. I've done a bit of research on the web and I'm getting contradictory answers. I have a project to develop where 80% area is admin and 20% frontend. I'm concerned how Google would treat the frontend, and whether it would be penalised.

I haven't even started it yet. Was planning on using the xalan transformation engine to output the HTML, although I have heard it can be a little slow. The reason why I am looking at this option, is that it would be very handy to convert the data in other formats other than HTML without having to recode... such as the ability to run a report and data could be in a format that is easy to import into an accounts application etc.
Safe bet...

I'd say your safe bet is to ensure your output is XHTML and not XML.

Search engines use your title / h1 / meta data tags and if your XML is output with custom tags I think the search engine will have a hard time understanding it.

Having said that I've never actually tested it, but I would say if its a big concern errr on the side of caution with XHTML.