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Currently with the netbeans platform source (and the full ide source), running "ant clean" requires downloading further jar files. This should be unnecessary in order to clean a project. I suspect the problem lies somewhere in nbbuild/build.xml or nbbuild/templates/common.xml. The task -cleanall eventually depends on -release.files which results in downloading external files.
First of all, this code branch is not executed if you have sources in the normal Mercurial repository; in that case the unversioned files are directly deleted, and module build scripts are not entered. To clean an individual module, it is necessary to know what files in the cluster it owns; and this computation resolves external downloads. <http://wiki.netbeans.org/ExternalBinaries> External files only need to be downloaded if your cache does not already contain them - which it would, if you had just done a build.
I understand it needs to know which files it owns, that makes perfect sense, but it shouldn't need to download files it doesn't yet have. The use case I'm thinking of here is when we're building packages for Debian, starting from the source zips. The first part of the debian build process is to clean the package. Debian build machines don't have external access, so can't download files, and in any case we wouldn't want to, since we use the versions of the library dependencies which are aready packaged for Debian.