The trial software can be used for a period of 60 days and they have all the features available in the retail Office 2007 software. During the download process, you will be given a Office 2007 serial number that will be required during the installation process.
There's no information whether these trial versions of Office 2007 can be coverted to full retail versions by purchasing a license. Atleast Office XP trial had that facility.
trymicrosoftoffice.com - Microsoft Office Trial Portal [via Patrick Schmid]
For those of you outside the US, you need to use the following URL's:
Update: Alpesh Nakars has confirmed that Office 2007 Trial version can be converted to a full retail copy by entering the Office 2007 serial number. You need not re-install Office 2007, just enter the registration key after the 60 day trial gets over.