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db4o being "open source" is just smoke and mirrors. Using db4o means (by their definition) all of your code is now GPL, which is the "GPL is viral" FUD that is usually attributed to microsoft and friends. Convieniently they offer non-GPL licences, that happend to be quite expensive. Repeat: You CANNOT use db4o in your production environment unless you buy an expensive licence. Read the legalese on their site before wasting time, and check their pricing.,

db4o is no different than almost any other dual licensed open source software (MySQL, Trolltech. etc) And with regards to price it's really affordable compared to popular DB vendor commercial licenses. What do you want? To use db4o for free in your commercial application and never give anything back to the community? db4o didn't invent the GPL, it was there long before db4o even existed and it's the most popular open source license.
db4o IS gpl no matter whether you like the commercial licensing policies or not.

This is simply not true. I can run 100 MySQL or postgreSQL servers in my production environment without any "commercial" licence at all. "Using" GPL software does not make anything you do with it GPL. An image edited with GIMP does not become GPL. Data stored into a MySQL database does not become GPL. Sending an SQL statement to a MySQL database does not make your application GPL.

The second you include the db4o jar into your application or reference any db4o object (i.e., the first line of code using it) does make your whole application GPL, according to the db4o people.
This has nothing to do with "not giving back to the community". It's nonsense and means this thing is not comparable to any truly open source database engine. It's a completely commercial project, and all those "blog posts" and "technical articles" about db4o are nothing but thinly veiled astroturfing, especially if all of them call it "open source" while conveniently forgetting to mention it's not usable in any released software without either a commercial licence or making it all GPL. That's why noone uses it.

As I said db4o IS gpl. If you think you're not violating the gpl go ahead and use db4o man!
On the other hand db4o commercial license is exactly as the OEM commercial license of MySQL:
http://www.mysql.com/company/legal/licensing/ (if you dig around the MySQL site you'll see that they monetize their database, there's a sales department and everything). You must understand that commercial and open source is not a contradiction anymore (you can have truly open source projects that have a company behind!!)
With regards to "noone uses it" I completly disagree. There're more than a 100 projects using db4o under an open source license (noone of these are paying for db4o):
http://projects.db4o.com