From all the smartphone platforms no one has fully upstreamed their port. There might be many reasons for that and I think the most commonly heard reason is the time needed to get it upstreamed. It is specially difficult in a field that is moving as fast as the mobile industry. And then again there is absolutely no legal obligation to work upstream.
For most of today I collected the ports I am aware of, put them into one git repository, maybe find the point where they were branched, rebase their changes. The goal is to make it more easy to find interesting things and move them back to upstream. One can find the combined git tree with the tags here. I started with WebOS, moved to iOS, then to Bada and stopped at Android as I would have to pick the sourcecode for each android release for each phone from each vendor. I think I will just be happy with the Android git tree for now. At this point I would like to share some of my observations in the order I did the import.
Palm's release process is manual. In the last two releases they call the file .tgz but forgot to gzip it, in 2.0.0 the tarball name was in camel case. The thing that is very nice about Palm is that they provide their base and their changes (patch) separately. From looking at the 2.1.0 release it looks that for the Desktop version they want to implement Complex Font rendering. Earlier versions (maybe it is still the case) lack the support for animated GIF.
Apple's release process seems to be very structured. The source can be downloaded here. What I think is to note is that the release tarball contains some implementations of WebCore only as .o file and Apple has stopped releasing the WebKit sourcecode beginning with iOS 4.3.0.
This port is probably not known by many. The release process seems to be manual as well, the name of directories changed a lot between the releases, they come with a WML Script engine and they do ship something they should not ship.
I really hope that this combined tree is useful for porters that want to see the tricks used in the various ports and don't want to spend the time looking for each port separately.