Here is a better version of the Display in Marked.app service which does not need any customization. This version which uses a shell script in Automator rather than an Automator action + AppleScript was originally written by Brett Terpstra, creator of Marked.app.
Posterous seems to have problems with my attached zip-file, so I have hosted it off site: →Marked.workflow.zip
A side effect of posting the previous post was that I wrote a OS X Service that takes the selected text, saves it as a temporary file and opens that temporary file in Marked
I have attached the Service. Just put it in ~/Library/Services
Update 2011-09-05: There is a new version available which you do not have to customize here.