According to Jani Pirkola, Project Manager with realXtend, the OpenSimulator-compliant virtual world platform, his team has added a feature that makes it possible to teleport from within realXtend directly into Second Life or another OpenSim world. To do this, you bring up realXtend's address bar, and type in an HTTP-style address. Typing in "rex://world.realxtend.org" for example will send you to realXtend's log-in URL. ("llMapDestination("sl://cisco", <76, 173, 142>, ZERO_VECTOR);", for example, sends you to Second Life.) That's quite a kludge of code, but here's the additional beauty feature: evidently you can create objects in realXtend that are embedded with links that take you to other worlds. When you click the link, the viewer brings up a log-in window; enter in the avatar name and password associated with the other world you're going to, and the teleport process begins.
Read more about it here. I haven't had a chance to check this out yet, but if it works as described, this is another significant milestone for interoperability. Combine this with a cross-world message bridge (which I only wrote about a few weeks ago) and you have basic multi-world connectivity. At this rate, we're bound to see even more breakthroughs before the year is out. Much mahalo to Pier at Koinup for the tip!
Update, 11:00am: Per Jani's clarification in Comments, corrected log-in code. Also in Comments, former Linden CTO Cory Ondrejka expresses disappointment that this process depends on a "'Trust us, give us your username and password'-antipattern that is seen so often on social networking sites."