Apparently, there's going to be a debate at WWW2004 about whether the Semantic Web will scale: "However, with only a few exceptions we noticed that current research and development is focusing on creating new technologies for facilitating the Semantic Web. Available technologies from other disciplines such as databases are rarely reused and adapted. Hence, most Semantic Web systems do not scale to Web-size problems.
Lately, several researchers doubted whether the Semantic Web idea will ever scale for numerous reasons technological [But03], theoretical [van02] and practical [MS03,Sow]. Dedicated workshops on that topic [CKDE03,VDC03] have been organized recently to promote research to improve scalability. We will pick up these three categories of doubt by organizing the panel in three parts discussing each aspect: theory, technology/implementation, and practise."
I'm not sure I agree, there are very few Semantic Web systems that don't reuse existing SQL databases - they just suck at storing triples. With Kowari, and I'm sure with other native stores, the data structures and techniques used are taken directly from databases. They mention "Is the semantic web hype?" (which I responded to) and a few others. Although, there's no links to syllogism, metacrap or gnomes. BTW, I'm still not sure why you'd want an XML version of OWL.
"Network round-trips are often considerably less costly than the time taken for a transactional database operation due to the need to forcibly log transactional operations which is very costly in terms of disk performance. i.e. network round-trips aren't always the performance bottleneck." From Martin Fowler's First Law of Distribution.
As long as you keep the Semantic Web like the Web there's no real reason why it shouldn't scale.