"Ubiquitous" = everywhere at once
"Ubiquitous web" = a web that learns and reasons, based on the anytime/anywhere/anymedia paradigm. You can access it from wherever, and all of your data and applications can be stored in it. It's "pervasive" because all physical objects are resources accessible by URIs. Every object is web-accessible and interlinked. It strives to provide you with the right thing at the right time in the right way.
Semantic web stuff includes: thesaurus and taxonomy, ontologies, personal assistants, semantic search and websites...
Ubiquitous web stuff includes: semantic social networks, semantic email, context aware games, semantic agents ecosystems, natural language...
Ubiquitous web agents are going to be more location aware than the standard browser. They are user-aware as well as context aware and users can also annotate their resources. It also allows for easier browsing because of everything being arranged in folksonomies, so you can jump from one concept to a related one. The social tags provide information on the proximity of the relationships.
The ubiquitous web is also "context aware" (location and user aware). A knowledge and communication model improves the user experience in the physical context, very much like the web does in cyberspace. "Context" allows for the rectification of the properties in the environment of the application. This means that it can determine what to customise and when.
The "adaptation rules" are specified by UML annotations in the "context" dimension. involves a set of generic and pre-defined operations for each model element which is part of the information, presentation, operations or navigation design.
So...the ubiquitous web can adapt the environment to user preferences. This creates a reactive environment.
There's a lot of work going on in mobile (see MORFEO) at the moment and also in cameras, GPS, search technology...
Check out the UWA|2008 - International Workshop on Ubiquitous Web Applications and the W3C group.