This should be fine, since you haven't pushed your commits anywhere else yet, and you're free to rewrite the history of your branch after origin/master. First I would run a git fetch origin to make sure that origin/master is up to date. Assuming that you're currently on master, you should be able to do:
git rebase origin/master
... which will replay all of your commits that aren't in origin/master onto origin/master. The default action of rebase is to ignore merge commits (e.g. those that your git pulls probably introduced) and it'll just try to apply the patch introduced by each of your commits onto origin/master. (You may have to resolve some conflicts along the way.) Then you can create your new branch based on the result:
git branch new-work
... and then reset your master back to origin/master:
# Use with care - make sure "git status" is clean and you're still on master:
git reset --hard origin/master
When doing this kind of manipulating branches with git branch, git reset, etc. I find it useful to frequently look at the commit graph with gitk --all or a similar tool, just to check that I understand where all the different refs are pointing.
Alternatively, you could have just created a topic branch based on where your master is at in the first place (git branch new-work-including-merges) and then reset master as above. However, since your topic branch will include merges from origin/master and you've not pushed your changes yet, I'd suggest doing a rebase so that the history is tidier. (Also, when you eventually merge your topic branch back to master, the changes will be more obvious.)
~ Answered on 2011-02-21 12:45:22