Many older scanners that were designed for Windows XP are probably not supported under Windows 7. Newer scanners that were designed for Windows Vista are more than likely supported under Windows 7. It is entirely up to the manufacturer as to whether they want to devote the resources necessary to develop and provide drivers (or not).
If you want to know if one of the devices you have currently attached to your system will work if you upgrade to Windows 7, I'd suggest you run the Windows 7 Upgrade Advisor. This software scans your system and reports on hardware and software compatibility.
For example, I found that my Epson Perfection 1200U scanner is not supported. Upon checking the Epson site, I found the following FAQ entry:
So, the answer to your question depends on what you have or what you are looking for.
Larry