Ok, I was looking up lambda calculus (damn you William) and I don’t understand why this never caught my curiosity until now, but where in the world did the B in Lambda come from? Google is no help other than some blogger ranting about it’s ancient correctness. Modern Greek pronunciation gives no clue either… I understand the whole double labial thing, but I am suspicious of the spelling reflecting any real world phonetic influence.
Oooh, Lambda, the Ultimate Calculus! (Sorry, insider computer geek joke.)
Wikipedia has this, supposedly cribbed from Smyth: “The ancient Grammarians and dramatists attest to a pronunciation of /laːbdaː/ (λάβδα) in classical times.”
If correct, this begs the alternate question, “Where in the world did the μ in Lambda come from?”
But the μ comes from the original name, doesn’t it? It seems that the name of the letter in the Semitic languages had the root l-m-d.
As a guess, maybe it was [lamda] to start with, because, if you say it pretty quickly, a seems to naturally pop in there – I couldn’t find an exact parallel but there’s μεσημβρία from μέσος + ἡμέρα where a appears as well, and then maybe the [mbd] was simplified to [bd] which was a lot more common than [md], and you get λάβδα. 
From Smyth http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0007:
“c. Labda is a better attested ancient name than lambda.”
I would be afraid to argue with Smyth, but I didn’t notice the Phoenician root letter before. I would guess that the ‘m’ sound in the Phoenician lamed became a ‘b’ sound somehow in Greek, but the spelling retained the μ. That’s sheer guesswork, though.
Well… So there is an ancient basis to this. I might have thought this was influenced by French pronunciation of Lamda.
Ditto on the Phonecian/Semitic origin:
http://www.hebrew4christians.com/Grammar/Unit_One/Aleph-Bet/Lamed/lamed.html
Hebrew has the letter’s name itself written with a long ‘a’ as in the Greek: lāmed.
I have one guess: that, for some reason, “β” replaced “μ” in *λαμδα → λαβδα, and that λαμβδα is merely an orthographic compromise of the older, more Semitic way, and the newer Greek way.
I’d be afraid too … but “better attested” doesn’t mean “original” or “first attested” 
. Perhaps λαμβδα was the scholarly form while λαβδα was the popular version. Although probably not.
Maybe another possibility is that it was through the influence of names like Λαβδακος and Λαβδα. It seems the ancients thought these names were derived from the letter, but their etymologies are often wrong and this could be some kind of later interpretation of the name. I couldn’t verify whether these mythological figure predate the introduction of the alphabet, so this is really based on nothing.
I forgot to ask last time: is the form λαμδα found in ancient times?