Now the instructions about how to download debian packages live on a separate
page of the wiki instead of on the front page or in the README. The README is
only about building from source. The front page contains pointers to those two
other pages (the debiandownloadpage and the README).
Note that using "whatever version of python the name 'python' maps to in the current shell environment" is more error-prone that specifying which python you mean, such as by executing "/usr/bin/python setup.py" instead of executing "./setup.py". When you build tahoe (by running "make") it will make a copy of bin/allmydata-tahoe in instdir/bin/allmydata-tahoe with the shebang line rewritten to execute the specific version of python that was used when building instead of to execute "/usr/bin/env python".
However, it seems better that the default for lazy people be "whatever 'python' means currently" instead of "whatever 'python' meant to the manufacturer of your operating system".
Zooko O'Whielacronx