Alongside the source code release, Google will be providing binary firmware for the Galaxy Nexus phone and the new Nexus 7 tablet (which already ships with Jelly Bean). This will enable open source developers to start working on custom ROMs for these devices immediately. Binaries for the Nexus S phone and the Motorola Xoom tablet are also expected to be released some time soon.
With Android 4.1 available in the AOSP project, the first ports based on this version of Android are likely to start arriving soon. The CyanogenMod project, which develops one of the most popular custom ROMs, has already stated its intention to begin work on a Jelly Bean version of their OS as soon as they have completed work on CyanogenMod 9 (which is based on Android 4.0 "Ice Cream Sandwich"). CyanogenMod 9 is currently in release candidate status.