In Go 1.2, encoding/gob respects the new encoding.TextMarshaler and
encoding.TextUnmarshaler methods, and net.IP adds those methods.

Unfortunately, this means that the gob encoding of a net.IP changes from Go 1.1 to Go
1.2. That by itself might be okay: I don't believe people expect that Go 1.1 will be
able to read Go 1.2 gobs.

However, gob is also a bit picky about the decoding: if the type has a TextUnmarshaler,
gob expects to get the TextMarshaler version of the encoding and does not accept an
encoding of the underlying data. 

A Go 1.1 gob encoding of a net.IP will send the gob encoding of a standard []byte. Go
1.2 gob will refuse to decode that form.

The only other type we have added a MarshalText or MarshalBinary method to is time.Time.
But time.Time already had a GobEncode and GobDecode methods, so its behavior with gob
does not change. It appears that net.IP is the only affected type in the standard
library.

Possibilities:

1. Do nothing; allow Go 1.1 and Go 1.2 to be mutually unintelligible in gob when there
is a net.IP involved.

2. Change gob to accept the concrete data representation during decode, so that Go 1.1
can talk to Go 1.2 but not vice versa.

3. Change gob to ignore MarshalText/UnmarshalText, allowing only
MarshalBinary/UnmarshalBinary.

4. Remove net.IP's MarshalText/UnmarshalText methods, breaking encoding/json's support
for net.IP (new in Go 1.2).

5. Replace net.IP's MarshalText/UnmarshalText methods with MarshalJSON/UnmarshalJSON, so
that we keep encoding/json working well for Go 1.2. (Of course, the new generic methods
were designed explicitly to avoid making package net mention JSON, so it is a bit of a
shame not to use them.)

I am leaning toward 5, which seems like the most limited change.

Rob, what do you think?