have you installed new themes ?
if you haven't yet , then install them and check if there are some other options than of these two themes .

digital nature = alpha
nokia nseries = beta

Changing to default themes won't fix what he's talking about. Please understand the issue more before suggesting fixes. ESPECIALLY since it has become obvious that he only has alpha and beta themes on his N900 from the posts made. Which means he has this issuue witht the stock themes. Which would've been obvious to anyone who has ever looked at the theme files that make up the lockslider. The actual "slide" part (I.E. the groove that the slider looks like it moves in) is part of the background. You cannot make a theme with normal (without making it look especially wierd in both portrait and landscape) in portrait mode and still have it look right in landscape.

Anyway, I'd like to see the CSSU include support for loading a portrait mode image as a background for when the lockscreen is in portrait, just as they did for App. Manager. Until then, a tech savvy solution might require modifying the method used here:

http://talk.maemo.org/showthread.php?t=72541

- Edit -
Looking in depth at the scripts that are involved in the image magic using fanciness I linked, I don't think what I said is too feasible. In fact, it's not doable as far as I can tell, so either someone recodes the closed source parts of systemui or whatever it is that contains the lockslider screen, or otherwise substitutes it with something else.