When it's official, mesh uploads will cost L$; how much will be a function of its complexity--but Oskar Linden assures all that accounts on the beta grid will have the L$ to be able to try out mesh.
So... fire up your 3D rendering software. (And if you don't have any, there's some that can be had for free.) We'll have to see just what people will be able to do with avatars, but I'm looking forward to seeing what happens, and seeing just how much mesh will improve the rendering lag situation.
UPDATE: Sand Castle Studios has been very much involved in mesh for SL, and have a status report. About the avatar issue, they say this:
Once rigged to the Second Life avatar skeleton, a worn mesh is able to move along with the movements of an avatar in-world. Rigged mesh objects can be worn as simple attachments such as jewelry, more complex objects such as clothing or hair, and can even replace the entire Second Life avatar if desired.
In its present state, the rigging system does not allow the creation of arbitrary skeletons or bone offsets (moving the joints around). Mesh objects must be bound and rigged to the default Second Life avatar skeleton AS IS. Essentially, this requires artists to build their assets around the skeleton, instead of building their assets and putting the skeleton into position accordingly. However, Linden Lab is presently working on adding these features.
So, right now, we're stuck with the existing avatar skeleton and its limitations, but maybe that will change. I hope so; perhaps the cheesy games that one has to play to create avatars outside the constraints of the stock avatar skeleton (and the accompanying inconsistencies) can be junked once and for all.