I have a slightly different take on this.
I have been a consultant for over 10 years and throughout my career, I have seen the term Solution Architect being abused so often. To me, there s no such thing as a J2EE Artchitect or a .NET Architect when it comes to Solution Architecting. Asking a technology expert to architect a solution is akin to asking a plumber to design a building. If someone is a J2EE specialist or a .NET specialist then, they better have hands on skills. That does not mean that they will have any architecting skills. There are a lot of other things a Solution Architect needs to consider besides technology and I would rather have someone who understands the Business problem, IT processes, can manage change and resolve conflicts because these are the areas in which real Solution Architects spend most of their time, every day. Most IT projects fail not because of lack of technical skills. They fail because there is no one who completely understands the technology + the business needs. In other words, there is no real Solution Architect in the team.
So I would go for someone who is a real Solution Architect and has a good understanding of what my business needs. He needs to be proficient in some of the technologies like .NET or J2EE, Database to filter out those candidates who have a very shallow understanding of technology. He should completely understand the technology behind his solutions from end to end. Whether that technology is J2EE or it is .NET is not so important.
To conclude, I would prefer someone with proven Solution Architecting skills + J2EE expertise over someone with .NET expertise and unproven Solution Architecting skills.