As most of you know, Sun acquired MySQL (and later Oracle acquired Sun), and during these acquisitions, there were a lot of FUD in MySQL community which resulted in creation of
Use MariaDB. They've just released a version lately. Maria engine is also much better than MyISAM. With opensource project, you go where the founder go.
If you're upgrading your databases anyway, you should consider other FOSS DBMSes, or data management software systems as well, not just MySQL.
For example, if your DB workload is analytics more than transactions, and data is mostly added in batches, you should consider using MonetDB, a columnar DBMSes which is about 1.5-2 orders of magnitude faster than MySQL when doing analytics-only.
Another option is PostgreSQL. It's more transaction-oriented - like MySQL; in fact, they're similar enough to be proper competitors. There are arguments for preferring PostgreSQL and arguments for preferring a MySQL variant.
On DB-engines.com you can find other potentially-relevant DBMSes, FOSS and closed-source/commercial.
Personally, I think that everyone who needs to set up a new MySQL installation today should be using MariaDB. MariaDB is based on the latest MySQL 5.1, actively maintained (including fixes for bugs reported against MySQL) and packages Percona XtraDB / InnoDB plugin.
I don't even know if "fork" accurately describes Maria, since they are committed to keeping up with current releases of MySQL.
In short: MariaDB 5.1 = MySQL 5.1 + Percona XtraDB + additional useful patches + active development and maintenance
Update late 2011- May 2012: I switched from MariaDB to Percona Server to get to MySQL 5.5 but I intend on switching back once the Maria team has a stable 5.5 based release. I think that everyone who needs to set up a new MySQL installation today should be running Percona Server. If you are using SSDs, you *must* run Percona Server.
2013-2018: Happily ran Percona server 5.6 this whole time.
2018: upgraded to Percona server 5.7 and then almost immediately to MySQL 8.0. [ At this point, I am not knowledgable enough to talk about the differences between MySQL 8, Percona Server 5.7 and MariaDB 10 and I can't say why you might want to choose one over the others ]
Google has a memory library patch out that supposedly improves performance on multi-thread / core mysql installations. Supposedly, the performance bottleneck was in the memory allocation routines, and would manifest itself starting at 4 cores. I'm currently running a dual proc box with 4 cores each, and am having no problems with 5.1.x without the memory library patch.
My biggest suggestion to you would be to install 5.4 and run a benchmark suite against it. If it does the job, go with it. The fragmentation of the various forks does suck, but maybe it will be for the best. At least the founders have money now to fund their own direction... I wouldn't take the advice to follow them blindly though... It took a lot of time and efffort to set up the mysql infrastructure initially, and while it wouldn't hurt to support the team that did it the first time, there is no guarantee they will succeed the second time. So... support them, but don't rely on them until you know you can rely on them.
Just an update on this. We chose to go with Percona and we can't be happier. Percona guys definitely know their stuff and keep up with the new technologies. Just checkout http://www.mysqlperformanceblog.com/ Top notch software plus top notch people and great support, you can't go wrong.
I think it's still too early to write off MySQL entirely. I'm not enthusiastic about Sun/Oracle gaining the reins, but I don't have a clear idea what direction they'll take with it. Regardless, I'd wait another year before seriously thinking of switching to a fork. Time will tell which project has the resources and motivation to survive, or if a fork is even necessary.