Can I create a text field which is both a autocomplete taxonomy, and a freetagging field I want to let my users store a list of their interests. I don't know what they are going to say (or how many interests) so I am going to let them create their own, preferably as one big comma separated list. 

But I'd also like to have those interests as taxonomy terms so that I can find users later on based upon their interests...

Can you suggest a way of doing that? 

It would be best if the field was autocomplete too to reduce the number of variants so for example I might type into my interests field

Books, Movies, Hitch

and if the system has other people interested in "Hitchhikers Guide to the Galaxy" then it offers that to me, along with "Hitchhiking", "Hitch (film)" and so on...

(In case you are familiar with it what I am describing is very similar to the LiveJournal interests field)

I am using Drupal 7, Profiles2, and MySQL. I might consider writing a custom module for this if that is the only way, but only if there is demand for it elsewhere.

Hey- thinking about it I'd quite like the way that this site does its tagging of posts..... :-)
That is how the autocomplete taxonomy widget works by default?

It provides autocompletion, but users can also add their own new tags.

Simply add a taxonomy field to your profile, select the Autocomplete Widget and you're good to go.