Pronunciation – How to Pronounce ‘cf.’

In normal speech, I would say "that is." I would not say "eye ee" for a couple of reasons.

• The audience may not actually know what "eye ee" means.
• It could be ambiguous because another term IE could mean something other than "that is". Consider a speech about web browsers. The audience would not know what you are referring to when you say "eye ee". Is that i.e. or is that Internet Explorer? The audience can probably come to the conclusion of which one it is, but for clarity, you should say "that is".
• Public speakers do not usually say i.e., so you should follow that convention.

While reading i.e. from text, I still say "that is." It is the same with e.g., I would say "for example." However, it is not incorrect to say "eye ee" since you, yourself know what the context is, it is fine.

To summarize, it is very common to say "that is". If you are reading aloud to listeners who are unable to see the text, I would recommend saying "that is" to avoid any ambiguity. If they can see the text and clearly know that you are conveying the abbreviation i.e, you can say "eye ee". If you prefer, you can say the full latin: id est

Essentially the same question was also posted on the linguistics stackexchange, several months after the posting of the present question here on the English stackexchange. As of Jan 14, 2016, here is what we have learned there:

1. At the present time, there does seem to exist a subgroup of adult native English speakers who pronounce the "th" as voiced. It is not clear if there is any underlying reason for this that would be common to all or even most of them. There do seem to exist some regional tendencies (see 2. below), but it seems there must be some other factors at play as well (see 3. below).

2. There indeed exists some published research on this: the Survey of English Dialects (Orton et al. 1962-71) recorded "a number of instances of the voicing of all four fricatives in word-initial (Anlaut) position in the South and the South-West Midlands" (Fisiak 1988); see Voitl 1988 (also here). Related research includes that by Martyn Wakelin, esp. Wakelin and Barry (1968) (thanks to Alex B. for all of this).

3. From the comments by musicallinguist, it seems that sometimes even among siblings raised in the same household (of native English speakers, in an English-speaking country) there can be a difference in the voicing of the initial "th," which then persists into adulthood. References to published research on this aspect would be most welcome.