The Math

Here is how the delegate math breaks down post Pennsylvania:

With a net gain of approximately 12 pledged delegates from PA (after winning by 9%),
Clinton has pulled within 154.

Clinton 1335
Obama 1489

There are 9 contests with 408 pledged delegates remaining. In order to have a majority of the pledged delegates, either candidate must get 1627.

Leaving Obama with 138 delegates needed, and Clinton with 273 delegates needed.

Since Democratic delegates are given out proportionally, Obama only needs to win about 34% of the remaining votes. Clinton, on the other hand, needs to about 67% of the vote.

That is virtually statistically impossible.

So, what's the next measure: Total delegates.

Including Super Delegates, either candidate needs to get 2025 total delegates to win the nomination. Clinton currently leads among supers, 256-234. That 22 point margin was much larger a few weeks ago, but Obama has closed the gap.
So Obama leads in total delegates 1723 to 1591.

Assuming neither candidate gets a swell of Super delegate endorsements in the next few weeks, Clinton again would need about 65% of the vote in the remaining contests to pull close to Obama in the total delegate count.

Finally, we have the overall popular vote:

Obama 14,417,619 (49.2%)
Clinton 13,917,009 (47.5%)

But if you count Florida, where none of the candidates campaigned (b/c FL broke the rules, moved their primary up, and were punished by the Democratic Party - and consequently, all teh candidates SAID they would not campaign there) but Hillary attended for a victory party:

Obama 14,993,833 (48.3%)
Clinton 14,787,995 (47.6%)

This makes Indiana extremely vital.