Gosh, I'm not sure. pdf-writer should be in vendor/gems, and I just tried running the tests with Rubygems version 1.3.1 and 1.3.5. I've been running Ruby 1.8.7 for a while now. Sometimes it seems that gem errors like this pop up because a dependency isn't loaded. Say: the "color" gem is a dependency of pdf-writer, and the color gem isn't installed. Though all the dependencies should be vendored, too …
Can you try rake --trace? Or maybe remove the config.gem "pdf-writer" line and see if that helps?
actionmailer (2.3.4, 2.3.2, 2.2.2, 1.3.6)
actionpack (2.3.4, 2.3.2, 2.2.2, 1.13.6)
actionwebservice (1.2.6)
activerecord (2.3.4, 2.3.2, 2.2.2, 1.15.6)
activeresource (2.3.4, 2.3.2, 2.2.2)
activesupport (2.3.4, 2.3.2, 2.2.2, 1.4.4)
acts_as_ferret (0.4.4, 0.4.3)
archive-tar-minitar (0.5.2)
authlogic (2.1.2)
capistrano (2.5.9, 2.5.2)
cgi_multipart_eof_fix (2.5.0)
curb (0.5.4.0, 0.5.1.0)
curl-multi (0.2)
daemons (1.0.10)
dnssd (1.3.1, 1.3, 0.6.0)
fastercsv (1.5.0)
fastthread (1.0.7, 1.0.1)
fcgi (0.8.7)
ferret (0.11.6)
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highline (1.5.1, 1.5.0)
hoe (2.3.3)
hpricot (0.8.1, 0.6.164)
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net-sftp (2.0.2, 2.0.1, 1.1.1)
net-ssh (2.0.15, 2.0.4, 1.1.4)
net-ssh-gateway (1.0.1, 1.0.0)
newgem (1.5.2)
nokogiri (1.3.3)
rack (1.0.1)
rails (2.3.4, 2.3.2, 2.2.2, 1.2.6)
rake (0.8.7, 0.8.3)
RedCloth (4.2.2, 4.1.1)
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rubigen (1.5.2)
ruby-openid (2.1.7, 2.1.2)
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rubygems-update (1.3.5)
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rubynode (0.1.5)
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