Back in the mid 80's I built a 'Serial-to-Speech' box which took normal ASCII encoded text over RS232 and played it as speech through a small loudspeaker.
It was very impressive, the words were usually easy to recognise, and if you had control of the text you could spell the words differently to get even better results.
The speech synthesiser was the SP0256A-AL2 mentioned earlier, but the clever part was another 40pin chip which took the serial and calculated the right codes to send to the synthesiser. The circuit, basically just a few ICs, was based on the application note in the datasheet. Without this chip and circuit I don't think I could have got the synthesiser saying anything sensible at all !.
The clever chip was called CTS256A-AL2, and was a pre-programmed PIC7041, from General Instrument Microelectronics, before they became Microchip. At that time the only datasheets I had for the chips were from Archer/Tandy/RadioShack, but nowadays there are much better datasheets on the web. With all the information available now I would think it would probably be relatively easy to code the 'text-to-speech' in a PICaxe. ( It was based on work done by the Naval Research Laboratories, so it must be on the web somewhere !. )