Decimal-to-binary converter for IP addresses
Code simplification and related:

Line 4: remove line 3 and replace line 4 with

ip_array = filter(None, ip.split('.'))


Line 7:
Create an exception, to prevent that a caller gets a string of error instead of the actual result.

Line 12: assuming this is not python3, it's better if you use xrange() instead of range(), because range would create an actual list listing all the items in specified range. xrange(), instead, returns the next item in the sequence without creating the list. This behaviour is the same as the range() function in python3.
Anyway, for your purposes changing it with:

for el in ip_array:
    ip_bin.append('{0:08b}'.format(int(el)))


or, even better:

ip_bin = ['{0:08b}'.format(int(el)) for el in ip_array]


Last line:
instead of appending the "." after each element, join over the dot:

return '.'.join(ip_bin)


Style:
Pursue consistency in all the code: if you use double-quotes for defining strings, use them everywhere, same thing with single-quotes (which i personally prefer for strings, reserving double ones for docstrings).

Reassuming:

def dec_to_bin(ip):

    ip_array = filter(None, ip.split('.'))

    if len(ip_array) != 4:
        raise NotValidIPException('Invalid IP Address format.')
    else:
        ip_bin = ['{0:08b}'.format(int(el)) for el in ip_array]
        return '.'.join(ip_bin)