You have to distinguish between spells with a casting time of 1 round (or longer) and spells which are modified by a metamagic feat and cast by a spontaneous caster.
A spell with a casting time of 1 round (like, for example, Summon Monster) takes a full-round action to cast and "comes into effect just before the beginning of your turn in the round after you began casting the spell“. The caster is indeed "busy casting" until her next turn and has to keep concentrating on the spell. Spells with an even longer casting time (like, for example, Regenerate (casting time 3 rounds)) require the corresponding number of consecutive full-round actions to cast.
Any distraction (most commonly: taking damage) that causes the caster to make a Concentration check might lead to losing the spell. If she takes damage from several attacks, she makes one seperate check for each attack - even if the same opponent hits her twice.
Spells with a normal casting time of 1 action (or less) which are spontaneously modified by metamagic feats also take a full-round action to cast but come into effect right on your turn.
Casting a Metamagic Spell
Sorcerers and bards must take more time to cast a metamagic spell (one
enhanced by a metamagic feat) than a regular spell. If a spell’s
normal casting time is 1 standard action, casting a metamagic version
of the spell is a full-round action for a sorcerer or bard. Note that
this isn’t the same as a spell with a 1-round casting time—the spell
takes effect in the same round that you begin casting, and you aren’t
required to continue the invocations, gestures, and concentration
until your next turn. For spells with a longer casting time, it takes
an extra full-round action to cast the metamagic spell.