Poor spelling is a massive problem, it's a far bigger problem than poor reading; secondary teachers would probably tell you that 60%+ of their pupils are poor spellers whereas only about 20% are likely to be poor readers.
Poor spelling really isn't a sign of dyslexia, if it were, 60% of the population would be 'dyslexic'! It's more a sign of poor teaching.
Go to this page: www.sounds-write.co.uk/pdf-downloads.aspx scroll down for the 2008 Research Report and read it. It reports on a very large sample of children taught with this particular phonics programme and shows that their spelling skills, measured by standardised spelling tests, are well ahead of those of the generality of children.
This is because spelling and reading are taught together, as they should be in any well taught phonics programme. (I qualify this with 'well taught' because phonics is frequently not well taught.) Children should get intensive practice in reading and spelling words containing the correspondences they are learning, in that way they develop automaticity in correct spelling, mostly through kinaesthetic memory (muscle memory). They also learn to look at words closely so they an use their observational skills to determine if a word they have written 'looks' right and, because they learn to read quickly, they read lots and become familiar with correctly spelled words.
What messes this up is schools which don't teach the two together, which don't give intensive practice in writing and which tolerate poor spelling (I'm not talking about early phonetic attempts in free writing before a child has learned all of the letter/sounds correspondences, I'm talking about when children have supposedly learned all the correspondences and still don't spell well). Which means that children automatise wrong spellings (muscle memory) which leads to the phenomenom of 'gets them all right in a test' (when they're thinking solely about the spelling) but doesn't get them right in creative writing, when, of course, they're not focussing on the spelling but on the piece they are writing and the automatic 'wrong' spelling just flows out!
You will shortly get marsha along to tell you about the iniquities of the English spelling system (if she isn't pounding away already). You may have great sympathy with her views but remember, children have to learn, right now, to spell under the system we've got.