I suggest you make a completely standalone rspec test until you find 
what the problem is. It Works For Me [TM].

$ cat ert.rb
require 'rspec'

describe "time" do
  let(:vtime_start){ Time.now - 10 }
  it "should work" do
    puts Time.now
    puts vtime_start
  end
end
$ ruby -rubygems ert.rb
Wed May 11 16:31:42 +0100 2011
Wed May 11 16:31:32 +0100 2011
.

Finished in 0.00109 seconds
1 example, 0 failures

$ ruby -v
ruby 1.8.7 (2010-06-23 patchlevel 299) [x86_64-linux]

$ ls -1d /var/lib/gems/1.8/gems/rspec*
/var/lib/gems/1.8/gems/rspec-1.3.0
/var/lib/gems/1.8/gems/rspec-2.5.0
/var/lib/gems/1.8/gems/rspec-core-2.5.2
/var/lib/gems/1.8/gems/rspec-expectations-2.5.0
/var/lib/gems/1.8/gems/rspec-mocks-2.5.0

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