I'm going to go against the grain here and say to follow through with the assessment and see what the outcome of that is. As a mum to a 4yo who is now in speech therapy I regret not doing something about it sooner instead of just palming her lack of talking off as just being slow to talk. Plus waiting lists are very long and the longer you put it off, the longer it will take to get some therapy happening if he needs it (which he may not, that's the whole point of having an assessment done) not to mention the longer it is left, the more intensive any speech therapy will be to correct it. Another thing is if you get onto it now, by the time he starts pre-school/kinder he should be on par with his peers and there wont be that obvious difference in speech ability.

Like Kat said, it's made very fun for the kids and it's very much play-based and most of the time they dont even realise that they are being made to do it kwim?

As for the speech issues and reading/writing development, my DD is well above average in that area. At her last assessment her speechie told me that she would be able to cope with starting school next year at 4.5 and do well, even though she isn't starting till 2010. She can also write her own name and read a little bit too (most of the reading has come from her 'homework' from speech therapy where we sit and practice what we've done in her therapy sessions) so even with speech issues, it hasn't held her back at all and she is very advanced in those areas for her age.