You really need to take your site offline - actually remove the files from the server and identify what type of hack this is.
First though I would suggest turning off javascript in your browser and visiting your page - do you still get redirected?
If not - then the problem is either:
a) a javascript file has been added to your site - or an existing javascript file has been edited. Examine all the .js files loading in the page.
b) an sql injection has added javascript directly into your articles (perhaps each and every article
Assuming you ARE redirected while is javascript turned off - then you are looking at either:
a) an edited .htaccess file redirecting you elsewhere
b) an edited (or 'included') php file setting headers and taking you elsewhere.
Are any of the add-ons in your site(s) listed here:
http://www.exploit-db.com/search/?action=search&filter_page=1&filter_description=joomla&filter_exploit_text=&filter_author=&filter_platform=0&filter_type=0&filter_lang_id=0&filter_port=&filter_osvdb=&filter_cve=
You need to know whether it is only your site compromised or whether it is other sites on the server, or even whether the server itself has been taken over, and that is a question for your host. Immediate re-infection after rebuilding from a backups could mean:
a) a cron job has been set up to re-infect you at a set period
b) another account on the server is infected and is reaching out to re-infect others
c) your site was compromised previously (dropped files within the site) but that these are sat dormant waiting for either an individual or a botnet to connect and take control.
d) or that the server is totally compromised and the hacker just reconnects to re-infect
There are some steps you could take - but frankly this is one area where if you need to ask it is probably a sign you aren't equipped to deal with the issue without expert assistance.
You could grep your files for likely file patterns c99, r57, web shell, eval(base64decode(, etc.
You could scan for files with recent creation dates or recent modified dates/times
Files changed in the last x days (1 days in this case)
find . -mtime -1
Files changed between two dates
find . -type f -newermt "2010-01-01" ! -newermt "2010-06-01"
You should scan log files for suspicious activity
You could download the files and let your anti-virus program scan them - this can give you a place to start (don't let it delete the files though as their contents can give further clues).
You should block access from known automated / scripted useragents (wget, libwww, etc)
All in all though you could spend days battling this with no guarantee of success. My advice would be to get some assistance from a Joomla security expert.