This link has a long- in-depth discussion of the upgrade.
We had a catastrophic melt-down of the entire Next Wave site yesterday by just upgrading one section – the WordPress blog. If you have a theme developed before 1.5 like we did- it may do odd things- so, BACK-UP EVERYTHING- not just your WordPress install- if it is a part of a larger site. We were lucky to have the Server back-up available for a reinstall.
If you just have a 1.5 WordPress install as your entire site- still do a complete back-up, including your MySQL database (our server has a back-up data base function in the CPanel)- and then proceed following the instructions above.
We still offer our clients the upgrade service for $49 and you can pay online- see the sign-up section.