I must admit I have deliberately waited before becoming involved in this thread as I wanted to see how popular the idea was before commenting.
I should also point out that there are several other older posts dotted around the forum that bring up a similar topic - I admit that they are mainly about a Talk Magic get together and not so much a convention, but nevertheless it's worth taking into account that nothing actually eneded up happening.
Now to the idea of organising a convention. I have some experience of this as I help organise the Ipswich Magical Society annual one day convention, and I can tell you that it takes an awful lot of work just to break even. If you want to have a good selection of dealers there then you will need to be able to guarantee a vast number of participants who are going to hang around all day. Whereas the bigger conventions can ask dealers for fairly sunstantial sums, the smaller conventions can not. In fact it is almost the other way around, just to encourage the dealers to come in the first place.
My suggestion would very much be along the lines of Goldfish and Happy Toad. Forget this idea for now until you know how many Talk Magic members are likely to actually attend. For sure arrange a meeting, but make it at one of the existing conventions, so that the dealers, lectures and show have already been arranged. THEN, If these meetings prove popular consider putting on your own convention. It is possible - it's just such a huge job that is going to take an awful lot of your time. (Plus the headache of worrying if enough people will turn up to make it pay for itself).
So why not arrange to meet at one of the following conventions? Blackppol of course would always be good, but there are usually many great conventions happening all over the country.
Top Secret Magic Day, Sun 5th June 2005. Milton Keynes
The Leeds Magical Association - 9th Convention. 14-15 May 2005
The Liverpool Mahatma Magic Circle's Day of Magic & Mayhem 22 May 2005
Barnsley Magic Day 12.6.05
The British IBM Ring Convention 27.9.05 - 2.10.05
Ipswich Magical Society Annual One Day Convention, Sunday 23rd October, Ipswich.
The names lined up at Ipswich so far include Jon Allen, Scott Creasy (BIP book), Scott Penrose and Joe Transini (although some are yet to confirm). I also should mention that already I know of at least 6 active TalkMagic members that will be attending the Ipswich convention (and more than a few who have said possibly).
Sorry to appear so negative - It really is a great idea, but I think it unlikely that we would ever get more than 20 Talk Magic members attend the same convention at the same time (maybe blackpool would be an exception).