OK, I’ll bite. Here are the Online Etymology Dictionary’s descriptions for each:
Speak:
O.E. specan, variant of sprecan "to speak" (class V strong verb; past tense spræc, pp. sprecen), from P.Gmc. *sprekanan (cf. O.S. sprecan, O.Fris. spreka, M.Du. spreken, O.H.G. sprehhan, Ger. sprechen "to speak," O.N. spraki "rumor, report"), cognate with L. spargere "to strew" (speech as a "scattering" of words; see sparse). The -r- began to drop out in Late West Saxon and was gone by mid-12c., perhaps from infl. of Dan. spage "crackle," in a slang sense of "speak" (cf. crack in slang senses having to do with speech, e.g. wisecrack, cracker, all it’s cracked up to be). Rare variant forms without -r- also are found in M.Du. (speken) and O.H.G. (spehhan). Not the primary word for "to speak" in O.E. ("Beowulf" prefers maþelian, from mæþel "assembly, council," from root of metan "to meet;" cf. Gk. agoreuo "to speak," originally "speak in the assembly," from agora "assembly").
[There was a very interesting parenthetical note that I took the liberty to underline for everyone…
]
Talk:
12c., talken, probably a dim. or frequentative form related to M.E. tale "story," ultimately from the same source as tale and replacing that word as a verb. E.Fris. has talken "to talk, chatter, whisper." To talk shop is from 1854. To talk turkey is from 1824, supposedly from an elaborate joke about a swindled Indian. Talk show first recorded 1965; talk radio is from 1985. Talking head is from 1968. Talkie "motion picture with sound" is from 1928.
So, there you have it. ‘Talk’ is related to ‘tell’ and ‘tale’, and ‘speak’ is related to ‘speech’ and ‘wisecrack’. 
In essence, it would appear that ‘speak’, having been primarily the verb of choice for use with large crowds, does derive from a more formal historical use. ‘Talk’ was the verb used with smaller groups.
However, I still stand by my statement that, in today’s use, ‘speak’ and ‘talk’ are basically interchangeable.
-Tim