AES-NI is the x86 instruction set from Intel (with cooperation from AMD) for speeding up encryption/decryption processes using the Advanced Encryption Standard. Intel has implemented AES-NI hardware support within their Clarkdale/Arrandale, Gulftown, Sandy Bridge, and future Ivy Bridge processors. AMD, on the other hand, won't be introducing processors with the AES instruction set until they launch the Bulldozer CPUs. The libgcrypt 1.5.0 release is the first to officially support the AES instruction set to take advantage of this capability on the latest Intel processors.
While support for AES-NI is new to the Gcrypt world, there's already been several notable projects already taking advantage of such instructions, including 7-Zip, TrueCrypt, dm-crypt, and OpenSSL.
Beyond the support for Intel AES-NI if support is found on the processor, libgcrypt 1.5.0 also adds support for Windows CE and EDCH. There's also support for OAEP and PSS cryptographic methods.
For more information and to see the other changes, read the mailing list announcement.