I don't remember 1.5 - 2.0 being a big deal, but some themes/plugins could need work.
Why not put up a test subdomain & install 2.2 there. Try the current theme you're using and see if it breaks.
Then, one at a time, try adding the plugins you currently use, testing each one as you add it.
If you do run into a theme problem, check the author's site to see if he put out a 2.x version.
In any case, I'd upgrade to 2.2 no matter what and do whatever it takes to work around it.
There are very good upgrade instructions at wordpress.org that cover the different versions.
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