The latest QuantLib release is the 1.2.1 version (September 10th, 2012.)
Debian users can find the latest version of the different QuantLib packages in the "unstable" (a.k.a. "Sid") distribution; see the Debian site for a list of available packages.
Source packages for all platforms are available from the SourceForge download page in a number of different formats. This also applies to eager Debian users not wanting to wait for the library to be packaged, or to advanced users wanting to configure the library with non-default settings. Compiling QuantLib requires a working Boost installation; Boost 1.34.1 or later is needed.