They said I couldn't have 5.0 ...

There are guides all over the web to upgrading your BB OS.

I upgraded because I was curious and had an idle Saturday night on my hands. Plus BB Messenger 5.0 installed on top of OS 4.6 on my shiny new Curve (8520) kept driving me insane. Every so often the cursor would disappear and I would need to do a hard reboot (battery pull or Shift-Alt-Del). The other participants of group chat would snicker at my typos and badly spelt swear words.

Anyway, make sure you have the latest version of BlackBerry Desktop Manager (free from blackberry.com) on your Windows machine (I think they have Mac versions, but the OS install files are all Windows AFAIK) and then back up all your data onto your PC.

Now I'll punt you over to a guide :-)
http://blackberrysync.com/2008/09/how-to-upgrade-your-blackberry-os/
(that deletion in Step 4 is very important)

It includes the link for all (?) mobile phone providers worldwide, so you can find your own. Or, if you're feeling adventurous you can select another provider -- but you have to pick the correct phone model! Bold OS on a Curve is a no-no!)

For example, my provider *cough* is still on 4.6 for the 8520, so I got another link off TechnoSpot in place of the file from Step 1 on the previous page:
http://www.technospot.net/blogs/download-official-os-5-for-blackberry-8520-released/ (Click "Download from Blackberry.com" below the ads.)

The file has the words "Wind Hellas" as that particular provider already approved 5.0 for their customers :-)

My backups didn't completely work (had to get added back into some BBM groups), but at least I got all my mp3s and contacts back. Ubertwitter reverted to non-paid version for a little bit but phoned-home and took the ads away in short order.

I also like the new "threaded SMS" in OS 5 -- check it out. It goes well with Facebook texts, so you can potentially carry on entire Facebook conversations without even logging into the site. (Now that's geeky.)