Well, if 1.5 doesn't add anything, then nothing will happen. Although, on the off chance that 1.5 will add something, then yes, something will happen.
Well, it looks like something is being added in 1.5.1, bug fixes, then i think it's safe to assume that something will happen
The pre relese came out today and it is just a bug fix for 1.5 but what i ment was will the server have to update again like this
It depends on the update as to whether your client and server version have to match. Often when it's only a small update/bugfixes you don't need to be on the same version as the server, but of course it depends on what Mojang changed and what kind of bugs they are. We'll just have to wait and see.