The implementation of single window mode in 5.5.2 beta 1 is broken, because it doesn't always honor modifier keys when clicking on links.
Example page:
http://www.perlentaucher.de/feuilletons/2006-12-06.html
All links in the main part of the page use "target=_blank" to open in a new window. Omniweb tab preferences are as follows: "Links to other sites open in the current window"; "Command-click to open links in a background tab".
Expected behavior: when command-clicking a link, it is opened in a new background tab (was so in OW up to 5.5.1, like the behavior of Safari or Camino)
Actual behavior: command-clicking a link replaces the current page.
If "Links to other sites" is set to "Work as specified in the web page", links with "target=_blank" are always opened in a new window, even when command-clicking or shift-command-clicking.
Modifier keys should be given priority over general preferences.
Update: fixed in beta 2. Thank you, OmniWeb team!