UPDATE: Hmph- now it's been marked Postponed. D'oh! I guess I'll just have to live with separate contracts and string coercion for the AJAX GET requests to deal with null values.
Cool- according to the comments on my connect requests (here and here), nullable support in WebGet/WebInvoke contracts is on DevDiv's list of "things we'd like to fix before RTM" (of Orcas). Being able to remove all those stupid conditional string->enum and string->int casts from my contracts will make all the time spent filing bugs and suggestions worth it!