This works for me on Hardy with no interesting repositories.  Having just installed apt-build, I did:

$ apt-build -v
apt-build version 0.12.31ubuntu1
$ sudo apt-build source mysql-server
You must put some ‘source’ URIs in your sources.list
E: You must put some ‘source’ URIs in your sources.list
Missing source package name for source_by_source().

(Then added source packages with the Software Sources tool)

$ sudo apt-build source mysql-server
-----> Updating package lists <-----
Hit http://archive.canonical.com hardy Release.gpg
Ign http://archive.canonical.com hardy/partner Translation-en_GB
Hit http://gb.archive.ubuntu.com hardy Release.gpg
Hit http://gb.archive.ubuntu.com hardy/main Translation-en_GB
Hit http://gb.archive.ubuntu.com hardy/restricted Translation-en_GB
Hit http://security.ubuntu.com hardy-security Release.gpg
Ign http://security.ubuntu.com hardy-security/main Translation-en_GB
Ign http://security.ubuntu.com hardy-security/restricted Translation-en_GB
Hit http://archive.canonical.com hardy Release
Hit http://gb.archive.ubuntu.com hardy/universe Translation-en_GB
Hit http://gb.archive.ubuntu.com hardy/multiverse Translation-en_GB
Hit http://gb.archive.ubuntu.com hardy-updates Release.gpg
Ign http://gb.archive.ubuntu.com hardy-updates/main Translation-en_GB
Ign http://gb.archive.ubuntu.com hardy-updates/restricted Translation-en_GB
Ign http://gb.archive.ubuntu.com hardy-updates/universe Translation-en_GB
Ign http://gb.archive.ubuntu.com hardy-updates/multiverse Translation-en_GB
Ign http://security.ubuntu.com hardy-security/universe Translation-en_GB
Ign http://security.ubuntu.com hardy-security/multiverse Translation-en_GB
Hit http://security.ubuntu.com hardy-security Release
Hit http://gb.archive.ubuntu.com hardy Release
Hit http://gb.archive.ubuntu.com hardy-updates Release
Hit http://archive.canonical.com hardy/partner Packages
Hit http://security.ubuntu.com hardy-security/main Packages
Hit http://gb.archive.ubuntu.com hardy/main Packages
Hit http://security.ubuntu.com hardy-security/restricted Packages
Hit http://security.ubuntu.com hardy-security/restricted Sources
Hit http://security.ubuntu.com hardy-security/main Sources
Hit http://gb.archive.ubuntu.com hardy/restricted Packages
Hit http://gb.archive.ubuntu.com hardy/restricted Sources
Hit http://gb.archive.ubuntu.com hardy/main Sources
Hit http://gb.archive.ubuntu.com hardy/multiverse Sources
Hit http://gb.archive.ubuntu.com hardy/universe Sources
Hit http://security.ubuntu.com hardy-security/multiverse Sources
Hit http://security.ubuntu.com hardy-security/universe Sources
Hit http://security.ubuntu.com hardy-security/universe Packages
Hit http://security.ubuntu.com hardy-security/multiverse Packages
Hit http://gb.archive.ubuntu.com hardy/universe Packages
Hit http://gb.archive.ubuntu.com hardy/multiverse Packages
Hit http://gb.archive.ubuntu.com hardy-updates/main Packages
Hit http://gb.archive.ubuntu.com hardy-updates/restricted Packages
Hit http://gb.archive.ubuntu.com hardy-updates/restricted Sources
Hit http://gb.archive.ubuntu.com hardy-updates/main Sources
Hit http://gb.archive.ubuntu.com hardy-updates/multiverse Sources
Hit http://gb.archive.ubuntu.com hardy-updates/universe Sources
Hit http://gb.archive.ubuntu.com hardy-updates/universe Packages
Hit http://gb.archive.ubuntu.com hardy-updates/multiverse Packages
Hit http://packages.medibuntu.org hardy Release.gpg
Ign http://packages.medibuntu.org hardy/free Translation-en_GB
Ign http://packages.medibuntu.org hardy/non-free Translation-en_GB
Hit http://packages.medibuntu.org hardy Release
Hit http://packages.medibuntu.org hardy/free Packages
Hit http://packages.medibuntu.org hardy/non-free Packages
Reading package lists... DoneReading package lists... Done
Building dependency tree
Reading state information... Done
Need to get 18.3MB of source archives.
Get: 1 http://gb.archive.ubuntu.com hardy-updates/main mysql-dfsg-5.0 5.0.51a-3ubuntu5.1 (dsc) [1422B]
Get: 2 http://gb.archive.ubuntu.com hardy-updates/main mysql-dfsg-5.0 5.0.51a-3ubuntu5.1 (tar) [17.9MB]
Get: 3 http://gb.archive.ubuntu.com hardy-updates/main mysql-dfsg-5.0 5.0.51a-3ubuntu5.1 (diff) [307kB]
Fetched 18.3MB in 1min14s (244kB/s)
gpg: WARNING: unsafe ownership on configuration file `/home/andrew/.gnupg/gpg.conf'
gpg: Signature made Thu 08 May 2008 14:54:16 BST using DSA key ID FA14013B
gpg: Can't check signature: public key not found
dpkg-source: extracting mysql-dfsg-5.0 in mysql-dfsg-5.0-5.0.51a
dpkg-source: unpacking mysql-dfsg-5.0_5.0.51a.orig.tar.gz
dpkg-source: applying ./mysql-dfsg-5.0_5.0.51a-3ubuntu5.1.diff.gz