from __future__ import *

The float printing options that ship with Erlang all suck. You either don't have sufficient precision, or you have an extremely verbose representation:

1> io:format("~w ~.18g ~g ~s~n", [0.1234567, 0.1234567, 0.1234567, float_to_list(0.1234567)]).
0.123457 0.123456700000000003 0.123457 1.23456700000000002548e-01
2> io:format("~w ~.18g ~g ~s~n", [0.1, 0.1, 0.1, float_to_list(0.1)]).
0.100000 0.100000000000000006 0.100000 1.00000000000000005551e-01

mochiweb includes an implementation of the algorithm from the "Printing Floating-Point Numbers Quickly and Accurately" paper in the mochinum module (which is standalone, if you just want to use it without the rest of mochiweb):

3> io:format("~s~n", [mochinum:digits(0.1234567)]).
0.1234567
4> io:format("~s~n", [mochinum:digits(0.1)]).
0.1

Problem solved :)