I have already seen a few of these available for hire -
http://www.friispray.co.uk and
http://www.yrwall.com - there are others too.
friispray is offering its software as open source and free to download. I have also been looking at using ArtRage 2.5 (3 coming out later this month) as it's got a minimal interface.
there's also
http://www.wiispray.com which is looking like it will become a full nintendo game. It has a spray can like housing for the wiimote
I'd like to make my own - using an IR lamp in a spray can. BUT I would really love to be able to do right click as well as left click... maybe a cobbled together solution attaching parts of a wireless mouse to the spary can? with just the right click button (anyone tried this?)
There's also the issue of software. The yrwall.com uses their own flash software with customise options - they're expensive to hire (£2500 a day +extras)
I have seen good source for Flash drawing things - open commons. I'm not a coder - but maybe someone out there is and could look at it?
I would love to work on this project with someone - I'm in Birmingham UK
http://www.nocircleno.com/graffiti/ Hopefull I can post URLS here!