YANBU. Constructive criticism is always better than piss taking, whether you're talking about spelling or anything really.
Spelling and grammar are hugely important, but as has been mentioned, some people are Dyslexic or have other learning difficulties. People make typos, spelling mistakes, grammatical errors all the time. Pointing them out is just irritating and pedantic. UNLESS critical meaning has been lost, in which case, why not be NICE about it?
I do think that on a public forum, where the written word is the MO, then people should make the effort. For example, I really dislike when people write a long post with NO PARAGRAPHS. I'm more than likely not going to read what you have to say, if you show such little regard for how you communicate to your reader.
Also: I absolutely, really, bloody, fucking HATE text speak. I think that goes without saying on Mumsnet though doesn't it...