h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Could a modern-welsh speaking person be understood, even vaguely, by a brythonic-welsh celt? How different have they become over the years?

What era do you mean by "Brythonic-Celt"? Brythonic is often used to refer to the Celtic language spoken in Britain before the Romans (p-celtic). However, I think it is also used specifically to refer to the latin-influenced language that was spoken during late and post-Roman times in Britain.
As a Welsh-speaker I have in front of me a poem (Y Gododdin) which was first written down in the 11th century but due to subject, language and reputation is believed to be originally written about 600 AD. It has words which are archaic or used in a different way to modern use. As a poem, it is obviously written in a particular way which is not quite the normal mode of speech. Some sentences are incomprehensible. However, some of the sentences and the whole general meaning is quite clear.
IF (and I stress if) this was the way that people spoke I would not be able to have a full sensible conversation with a 600AD Brythonic speaker. We would however be able to understand a fair amount of each other's speech. Many words would be similar or identical to each other. So, a conversation is out but communication although hard would be possible.

As I understand it the last great change to Welsh was at the point when Brythonic emerged as a Latinised Celtic in about 400AD. Since then the language has only changed gradually over time, becoming Modern Welsh. (Similarly, I believe both Cornish and Breton have only GRADUALLY changed over time). It has not experienced a massive quick change such as English did following the Norman invasion.

Any real experts out there?!

I remember reading a 16th century elegy for GCSE Welsh Lit. class. Although it was deeply moving - a father's lament for his young son - it was extremely different from modern Welsh and I wouldn't have had a clue if I had found it by chance!

Aberystwyth Uni. Welsh dept. are pretty good - try having a look at their site for more info. www.aber.ac.uk

I'd love to find other Welsh speakers on here...
By the way, shw' mae 'da ti, Huw B?

ansawdd!

regarding Welsh and Breton, I lived in Brittany for a short while many moons ago (when I was about 4) and my family rented a small cottage from a local farmer, who spoke Breton. My father says that the languages were vaguely similar and that they could sort-of understand each other, as long as they were talking about weather or animals (or simple, nature-based stuff like that), but that was about all.

I've heard Breton described as being like the result of a French speaker learning Welsh from a book. They are not totally incompatible, it seems.

There was a fantastic demonstration of how similar they are on the first floor of the Celtica Heritage Centre in a fine old building on the edge of Machynlleth, around 1995/96. There was a series of recordings which played speakers counting from 1 to 50 (or thereabouts) in Irish, Manx, Scots Gaelic, Welsh, Cornish and Breton.

Try here:
http://www.celticawales.com/

I know I can sit down and read old scotish...and old irish farly well. But then I have also studied the celtic from pre-history on...so I cheat. I can speak Gaelic better than I can translate it though.