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MacPorts recently started fetching prebuilt binary packages form packages.macports.org, which fails if there are none. This might happen for licensing reasons and currently always happens on Lion, because we're not building packages for Lion yet. This is what the line "---> Attempting to fetch" is about.
If this step fails, port continues to download sources and build from source like it did before, which apparently fails for you, because your download is corrupted. I can not reproduce your problem and successfully fetched the source tarball from ftp7.de.postgresql.org. Try running port with -d to see where your MacPorts is fetching from.
Checksum problems are a frequently asked question, have a look at FAQ and MisbehavingServers.
I have verified all mirrors we have listed for the postgresql90 port and they either server a correct error message, don't work at all or serve the correct file.
