Since strings cannot reliably be converted to floats and back, plistlib should provide an option to treat the real datatype as strings/data or to use the Decimal library class. Currently, reading and then writing a real value can change its representation in the plist even if it has not been modified. For instance:

Read:
<real>0.015</real>
Written:
<real>0.014999999999999999</real>

or 

Read:
<real>0.4</real>
Written:
<real>0.40000000000000002</real>

This should no longer be an issue on most platforms as of Python 2.7 and Python 3.1.  Both added a new algorithm such that "the repr() of a floating-point number x now returns a result based on the shortest decimal string that’s guaranteed to round back to x under correct rounding".  The new float repr applies to plistlib as well.  Here's an example:

$ cat pl.py
from plistlib import readPlistFromString, writePlistToString
d1 = dict(a=0.15)
s1 = writePlistToString(d1)
print(s1)
d2 = readPlistFromString(s1)
print(d2)
assert d1['a'] == d2['a']
print(d2['a'].hex())

$ python2.6 pl.py
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>a</key>
	<real>0.14999999999999999</real>
</dict>
</plist>

{'a': 0.14999999999999999}
0x1.3333333333333p-3
$ python2.7 pl.py
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>a</key>
	<real>0.15</real>
</dict>
</plist>

{'a': 0.15}
0x1.3333333333333p-3

Note that the binary representation of the float is the same on both 2.6 and 2.7 in this case anyway.

For more info, see:
http://docs.python.org/py3k/whatsnew/3.1.html#other-language-changes
http://docs.python.org/whatsnew/2.7.html#other-language-changes