I have been meaning to post my experiences with a generic data plan for ages and your question finally made me do it. Please forgive the length of the post but I thought I compile my journey in the hope to assist others a bit.
I am in Australia too using an Optus reseller and I can definitely confirm that running a BB without a BB data plan (they are hugely expensive here) does work very nicely. I have also helped friends run theirs, one on Vodafone, one on Telstra Prepaid and one on Virgin. But - it took me quite a bit of time and searching and trying around to get it all going. Its really not that tough, except for that the info seems to be out there, but it is scattered all over various posts.
Background: I am a long time corporate BES user and used to love BES with no end. It is just brilliant and it makes the BB such a great business tool. As part of a complete change in my life, I have no longer a need for BES connectivity.
I still like being connceted via email where ever I go (can't completely let go yet
) and it boiled down to the fact that I would be be perfectly happy with just using the Google Apps mail client for my mail, Google Sync for synchronising my Google Apps calendar with my BB calendar / contacts and, of lesser importance, some chat program getting me away from expensive SMS conversations with people.
Anyway, all the applications I mention above do not actually need a Blackbery Plan at all. A generic data plan will do nicely and they are much cheaper (at least here in Oz). Cheaper by around a factor of 100 on a per MB basis - it is amazing.
So here are the challenges I had and how I overcame them. If you are planning on ditching the BB plan, you must read this post:
HOWTO: Use BlackBerry on Generic NON-BlackBerry Data Plan
Mark Rejhon needs to be given a Pulitzer equivalent for this post. It basically contains all there is to know and he links to all the needed informaton. I know it has been done over and over, but let me join the long list of people who are extreely thankful for this post. Thanks Mark! I could basically stop here as Mark's post has it basicaly all.
My first problem started (I had not found Mark's post at that time yet, only a scaled down version of it at blackberryfaq) when I had my BB plan taken off my account. As expected, the GPRS in the top right corner turned into a lower case gprs. I did also expect that the BB built in mail client, BB Chat and the BB Browser would stop working (and they did). I did not expect for third party apps like my Google Apps mail client, Google Maps, Google Chat and Google Synch to stop working. But they did. Ouch!
Optus's helpdesk is not that much in tune with the fine details of how the Blackberries work and what the difference is between the BB data plan and a generic one. By the way, in Australia we have currently three providers offering Backberry data plans (Optus, Telstra and Vodafone - ok, and "Three" offeres some but only for the 87xx models when I looked last) - and all of the helpdesks that I called told me that a Blackberry handset will only work on a Backberry plan (which is sort of true). They also however told me that normal (as in generic) gprs or edge or G3 data usage is not possible on a Blackberry - and that is definitely not true.
So, after reading Mark's post, the most important thing is (as in Mark's post) to set the APN for your carrier. For Optus I know it is "internet" and no username and password. For Three, I don't know, but a quich google search gave me "3netaccess" as a suggestion. You may ant to try that.
Now, it is important that you do pull the battery out after putting the APN in and giving your BB a reboot like that. Then start the Gmail client (or Opera Mini (which I now use as a browser) or Googly Sync or whatever. I could not get any to work without the battery pull. So, set APN. Pull Battery. Then try.
Getting applications onto the BB without a working browser is not that easy either by the way since you cannot do any OTA - by definition. But Mark's post clears that up too. I now have a brand new Bold and with that one I just hooked up to my home WiFi and installed all the Googls apps and Opera and Google Maps and Beejive etc OTA like that. Much easier.
I so very much hope that this helps others. I sure love my BB - without the expense of a BB plan. With Google Sync I can even share a calendar between my wife and myself - that was only possible with BIS / Exchange and all the easociated expenses and hassles. Now it works much better and is free.
No Apps work, have dataplan but no BIS/BES. 
Thanks button.!! 
