There has recently (late 2010) been some debate on the pod_publishers list about whether Scribus could be used to make covers for books that Lightning Source (LSI) would print in high quality. The conventional wisdom was that unless you used exactly the mechanism suggested by LSI, which involved creating a cover in Indesign using the LSI-supplied template and then converting it to a PDF using Adobe Distiller, LSI would rasterize your cover before feeding it to the printer. This extra rasterization would result in subtle (but annoying) artifacts and lower quality printing overall. The most noticeable artifact is probably the "jagged bar code", although even that is hard to notice without terrific lighting and/or magnification.
I had read about this double rasterization problem in Aaron Shepar's book POD for Profit. But I really wanted to use Scribus anyway (it is free and runs on Linux, unlike InDesign which costs a lot and only runs on Windows and Mac). So in late 2008 I created a cover images, submitted it to LSI, and hoped for the best! This was for my book Nmap Network Scanning. Unfortunately, the result had the telltale rasterization artifacts, including the jagged bar codes:
Well that was a bummer! So I redid the whole cover with InDesign and the LSI template, and the result (at least the bar code) looked marvelous:
A couple years later, I saw John Culleton evangelizing for Scribus on the pod_publisher list. He even wrote a $5.95 ebook called Create Book Covers with Scribus. People asked him about the rasterization problem, and he claimed that his books looked totally clean.
After my experiences with Scribus, I was skeptical. But John agreed to send me a copy of his (published by him, not written) book Death Wore Black. He noted that he created the cover with Scribus and printed it with LSI. When I received the copy (thanks, John!), I immediately looked closely at the bar code:
Wow! The bar code looks perfect! But what about other artifacts? Maybe LSI replaced the bar code with their own after rasterizing or something. I decided to look closely at a couple words of cover text on the three books:
My Scribus Cover: 
My InDesign Cover: 
John's Scribus Cover: 
As you can see, the text in my Scribus cover is a but lumpy (look at the curves in the 'd' and the 'b'). But my Indesign copy looks fine, as does John's Scribus copy. So it is clearly possible to produce clean covers with Scribus which don't get rasterized by LSI. I'm not sure if this is because LSI improved their systems since late 2008 when I tried it, or because of the technique John uses (described in his ebook, which I have not yet read).
In conclusion, Scribus can product covers which are every bit as good as those using LSI's standard template and Adobe tool process. I hope to try it again with better results for the next edition of Nmap Network Scanning.