Avid say the following on their website:

"Version Protection

Any Media Composer v3.x, who purchased a new system since February 1, 2009 is entitled to a free upgrade of Media Composer v3.5.   This offer will be available until March 31st, 2009.   Customers who are eligible to receive this offer must act before March 31st, 2009.  All customers will be required to register their system before downloading their software. Customers entitled should visit www.avid.com/protect "

At the time of writing this email newsletter we do not have the exact price for a 3 to 3.5 upgrade if you do not qualify. It appears to be the same as the price for upgrading from any older version of Media Composer, i.e. £300 +VAT. However if you do qualify make sure you act now!