Looks like he raised just under $22 million in May, about equal to McCain's take.
Is that something to worry about? Maybe, but I really doubt it. Obama probably got a nice fundraising boost just after May 6, when he effectively wrapped up the nomination. But I'm guessing fundraising dropped off in the second half of the month, with the contest basically over but Hillary still in the race.
By contrast, I'm sure he got an even bigger bost in early June, between Hillary's endorsement and the general election beginning in earnest.
Anyway, even speculating about these ups and downs is probably overthinking it. In the past several days, Obama has opted out of the public financing system and made an "audacious" ad buy. I doubt the campaign would have done these things if, in the first case, it wasn't supremely confident of its overall fundraising ability and, in the second, June wasn't shaping up to be a pretty good fundraising month.
Update: A shrewd reader wants to know how much of McCain's $21.5 million haul in May was raised for the primaries and how much was raised for the general. (Any amount larger than $2,300 from a given individual must be saved for the general, since you can't contribute more than $2,300 to a single candidate in the primaries.) It's a great question given that McCain plans to stay in the public-financing system this fall, meaning he'll have to refund all of those general election contributions. That probably leaves his real total for May well under $21.5 million, and his cash on hand well under the $31.5 million he reported at the end of that month.
Update II: The Politico's Jeanne Cummings says the cash on hand figure is for the primaries, per the McCain campaign's disclosure.
For what it's worth, I'm not sure you can verify this stuff independently without tallying it all up yourself, contributor by contributor. The FEC summary pages report the overall cash on hand number, not cash on hand for the primaries. (So, for example, the Obama summary page says he has $43 million in cash on hand, even though the campaign has said roughly $10 million of that is for the general.)
If anyone out there has expertise in campaign finance disclosures, by all means feel free to weigh in on this.