The virus might mutate between now and the winter - indeed, it would be suprising if it didn't - but having a prior exposure to a milder variant is still likely to confer some protection. One of the notable features of this particular pandemic is the age range of those most seriously effected - mostly young.
It is thought that this is because older folks have been exposed to a variant of this particular H1N1 which has offered them some protection this time around, but has been absent for the last 40-50 years or so.
For the same reason, a vaccine made to combat this particular version of H1N1 will be effective against any mutation, although not 100%.
So, on the surface, the idea of having a "swine flu party" now whilst it is thought mild has some attractions - The reasoning being that it will be useful like chicken pox parties were.
There has been a lot of anecdotal chatter on bulletin boards discussing holding this sort of party, so it is probably worth offering a warning against - principally because it will aid the spread of the virus,.'Flu (not common colds, which most people typically confuse with 'flu) has some serious effects which no one should wish on their kids, and can be fatal - so on balance, not a good idea.