method xml.etree.ElementTree.tostring from module returns type bytes.
The documentation reads
"""Returns an encoded string containing the XML data."""
(from http://docs.python.org/py3k/library/xml.etree.elementtree.html#xml.etree.ElementTree.tostring as of 2011-01-18)

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Here is a test program:
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#!/usr/bin/python
# created for python 3.1

import sys
print(sys.version) # for help verifying version tested
from xml.etree import ElementTree

sampleinput = """<?xml version="1.0"?><Hello></Hello>"""
xmlobj = ElementTree.fromstring(sampleinput)
type(xmlobj)
xmlstr = ElementTree.tostring(xmlobj,'utf-8')
print("xmlstr value is '", xmlstr, "'", sep="")
print("xmlstr type is '", type(xmlstr), "'", sep="")
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test program output:
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3.1.3 (r313:86834, Nov 27 2010, 18:30:53) [MSC v.1500 32 bit (Intel)]
xmlstr value is 'b'<Hello />''
xmlstr type is '<class 'bytes'>'
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This cheap "fix" for this bug may be simply be a change in documentation.
However, a method called "tostring" really should return something nearer to the built-in str.