Note (added Oct 26, 2011): This answer is somewhat outdated now. See comments.
Xcode 4 (as of time of this writing) does not support IBPlugins at all.
You'll need to install Xcode 3 and its Interface Builder to edit nibs with IBPlugins.
(Be careful installing Xcode3 though, as it might overwrite your Xcode4 unwantedly)
So in short: use Xcode3's IB for editing IBPlugin-infected nibs and Xcode4 for al the rest.
To my knowledge there has not been a public announcement/statement gegarding IBPlugins and whether to still stick to them, or not. ast time I checked (day after Xcode4's release) documentation on IBPlugin still existed and I couldn't find any mark on deprecation (or strangely enough not even Xcode4 incompatibilities) either.
For more info, let me quote from Martin Pilkington's great Xcode4 review:
Loss of IBPlugins:
For all the cool new
hotness in Xcode 4, there is one big
gaping hole in its functionality.
IBPlugins cannot be loaded and as such
any NIBs that rely on components from
them cannot be edited. They can be
compiled, but only if you have Xcode 3
and IB 3 still installed. This is a
major missing component, though it
isn't unexpected. A line has to be
drawn at some point in order to say
"this is the first version we'll ship"
and IBPlugins are likely one of the
things that didn't make the cut for
4.0. Of course the big issues with IBPlugins is they don't work with iOS.
My hope is that they're missing
because Apple is working on an
improved system and would rather leave
it out of 4.0 than ship a half
implemented solution. That said I hope
they don't completely replace the
IBPlugin system as it is incredibly
powerful and one of the nicest plugin
systems I've coded for.
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