Obviously, there are almost certainly some people left, who would consider it impolite to be addressed by uncapitalised pronouns. However, I would judge the risk of this happening to you very low for the following reasons:
- This rule did not change without any reason: The default formal address is Sie (which is strictly capitalised anyway for other reasons), and if you are addressing somebody with du, you already are on more friendly terms with him, most probably dropping some other etiquette on the way.
- Though I capitalise these pronouns, I know only a handful of persons to do so as well and I consider myself to have performed a lot of written formal communication using du (it may very well be that some of these people only capitalised du because I did first). Therefore I expect that almost any other capitaliser of du is used to be addressed with an uncapitalised du and will only have few friends, if he considers this an issue.
- As already mentioned, you usually address somebody with du in formal communication only, if you already got to know him a little, and had the chance to notice some indicators that he might be offended by an uncapitalised du. For example, if you know that the person is an ardent advocate of the old spelling rules, has conservative views on etiquette and communicates very little in written form with people that are not of his kind; then you might rethink the capitalisation issue. However chances are very high that you will never communicate with such a person, let alone using du.
- Finally, if German is not your native language, people are even more likely to ignore this issue – if it is an issue to them at all.
As already noted, since the reform of the reform of 2006, Du etc. may again be capitalised in letters (§ 66 E). I think it is safe to say that letter was not intended to be taken literally, but also to include e-mails for example.