HP Calculator Forums
The main discussion forum for HP calculators is the Usenet newsgroup comp.sys.hp48. For those who haven't seen it before, comp.sys.hp48 is the best place on the Internet for discussion about HP calculators. News about new programs is also available, as well as technical information about the calculator. If you have a tough programming question not answered in the owner's manual or the FAQ, chances are you'll get an answer here. Comp.sys.hp48 is a very friendly "community", and unlike many newsgroups, it gets almost no spam.
If the above link doesn't work, ask your ISP for information on configuring your newsreader. If your ISP doesn't serve comp.sys.hp48, you can read it on the web at Google.
There is also a popular web forum for discussion about HP calculators, on the HP Museum. It is more dedicated to older models, but there is also some discussion of the graphing models. For Italian language speakers, HelpCalculator.TK is the most active.