To answer the problem of transistional forms with the response that groupings are determined by scientists and are therefore of no consequence, is a red herring.
But it's NOT a red herring. Not at all or in any way!
I'm going to give you an analogy. People age throughout their lifetimes. We classify different stages as child, adolescent, adult, middle aged, elderly, etc.
Now, NO ONE goes to bed one night as an adult and wakes up middle aged (although drinking to much can make you feel that way!). However, we can look at people and classify them into these categories.
What many creationists are saying is that for aging to occur I have to go to bed one day when I'm a child and wake up as an adult the next day.
Species is just a way for us to classify a GRADUAL process.
I recommend you read Parasimonium's thread titled "All species are transitional". Evoltuion is not focused on changes of of species by changes in POPULATION Groups.
If a population group A exists all members of A will always be withing that group A and give birth to members of group A. Now, if we had an external observer who could look at group A 1 million years ago and group A today he would see 2 different species but it would still be the SAME POPULATION GROUP!