No, Trump is not the official nominee until the Republican National Convention says so.
A number of things could go wrong (or right, depending on your politics) to prevent Donald Trump from becoming the Republican party presidential candidate.
The first thing to realize is the rules for "pledged" delegates vary by state. In the Republican Party system most delegates are "bound" by their states; their vote will go to their pledged candidate in the first round. If no one gets a majority in the first round, states begin to release their delegates from their obligations for the next rounds of voting. At that point it's chaos and delegates become free to make their own choices. Formerly bound delegates will become free to vote for any eligible candidate in the second round or later rounds. Eligibility is determined by party rules and can be changed at the convention. This is a simplification of a very complicated process, more detail below.
The second thing is that selecting a presidential candidate is a party process, not a legal one. The Republican Party decides how they select their candidates, and the Democratic Party has their own. This is further split up state by state. This is part of why the process is so weird.
That said, barring serious party shenanigans, if Trump gets the magic number of pledged delegates he is in... except for below.
He could die.
A similar situation happened in the 1872 presidential election when Liberal Republican candidate Horace Greeley died after the Electorial College had been chosen but before they had voted. All but three electors switched their votes to other candidates. This didn't throw the election, Grant already had it wrapped up.
What happens to Trump's pledged delegates if he dies varies state by state.
He could not get the necessary bound delegates.
Candidates typically don't "drop out" of the race, they "suspend their campaigns". Even if Trump is the only one on the ballot it's not assured he will be awarded the delegates. The rules vary from state to state.
If Trump fails to get the necessary bound delegates in the first round, voting goes into a second round. This is known as a contested convention and it's what Ted Cruz was hoping for. At this point things begin to unravel. The rules get complicated, both state and convention rules. Once unbound delegates can vote for anyone they want, or not vote at all, though it's unclear what votes will be counted. We'll get to that.
With Cruz out this is extremely unlikely. What is more likely is attempts at the convention to take bound delegates away from Trump with various procedural shenanigans, more on that later.
He could drop out.
I was waiting for Trump's April 1st press conference to declare his candidacy the greatest April Fool's Joke of all time. Alas, it didn't happen. But if it did, things would be in turmoil and the other candidates would come racing back to the campaign trail.
Republican National Convention shenanigans.
The 2016 RNC rules are currently not known. Yes, this is a little crazy. Just like Trump is the presumptive nominee, everyone is operating under presumptive rules.
The rules of the 2016 Republican National Convention are not law. Each convention decides its own rules. It's customary to adopt the rules of the previous RNC, and the tweaks since, but this is not required! Several key rules could be changed, or their interpretations changed, to "unbind" delegates, or change how votes are counted, or to allow bound delegates to simply not vote. Rule 16 and Rule 40 are the two big ones outlining how delegates votes are counted, and who is eligible for nomination.
Under the presumptive Rule 40, only Trump and Cruz are eligible for the nomination because they're the only ones who can "demonstrate the support of a majority of the delegates from each of eight (8) or more states". This rule can change. It was added in 2012 to prevent spoiler candidates, but that's exactly what the 2016 anti-Trump people want.
The presumptive, and heavily revised since 2012, Rule 16 deals with rogue delegates. Long story short, delegates cannot act against their state's rules. If they do they are considered to have resigned and voted according to their state's rules. Like many parliamentary systems, there's some wiggle room in the rule which allows the Secretary Of The Convention to pick and choose which violations they pay attention to. And, again, Rule 16 may change.
It's complicated. Pulling the presumptive rules apart is its own answer. Even then they might not be the 2016 rules.
Such shenanigans are a part of every RNC. An overt move would shatter the Republican Party and the people's already shaky faith in their nomination system. Expect a lot of rules lawyering. This is the big unknown.
A similar situation happened at the 1912 RNC to protect President Taft's nomination as the incumbent from the popular former president Teddy Roosevelt. Denied the Republican nomination, Teddy ran in 1912 as a 3rd party candidate.