OmniWeb 5.1.3 and 5.5sp3 displayed blocked ads as holes in a page layout -- a waste of space but not very distracting.
In place of a blocked ad, SP4 is displaying error text like this:
Cannot Load Address
Stopped loading <filtered:http://ad.doubleclick.net/adi/sitename/homepage;tile=1;dcopt=ist;sz=728x90;ord='+%20ord%2 0+'?>: Cannot show URL
I much prefer the empty ad holes.
[copy of my formal bug report]