An online citation index is available.
The American Physical Society is exploring the use of online publications. Starting in July 1995, Phys. Rev. Lett. will be online with Online Computer Library Center. (Currently this includes the Applied Physics Letters.)
The Division of Plasma Physics is also looking into using the guerilla preprint service offered by Paul Ginsparg. Here are the contributions in plasma physics.
There is an effort underway to create HyperTeX, a version of TeX with hypertext capabilities. This will allow interal links (e.g., finding Equation 20) as well as external links (finding an online citation).