With the Mike Wilks signing, the Wizards are ever so close to the edge, per the Daily Dime:

Now, if Daniels doesn't get healthy by then and the Wizards have to sign him for the rest of the year, then they would go over the cap.  Thankfully, it looks like AD could be back as soon as Saturday so it probably won't come to that.  But why won't Abe go over the tax for a better solution?  Well, it might surprise you to know that Abe has never told Ernie that he can't over the tax.  From Wizards Insider:

So why won't Ernie go over?  It's a complicated, but rational reason.  Let's start by addressing what would happen if they went over.  I'll let Ivan Carter describe how it works (this was written before Daniels' injury and the subsequent signing of Mike Wilks so the numbers are slightly off, but you get the general idea):

Now knowing that, let's say that Ernie wanted to sign a more permanent solution like Earl Boykins and they agreed to a one year, #2 million deal (which would be a huge bargain considering he's looking for $5-6 million a year after opting out of a contract where he was making $3 million a year, but this is hypothetical after all).  In signing Earl, the Wizards pay the $2 million for his salary, plus about another $2 million for the dollar to dollar tax.  On top of that, the Wizards lose out on about $2-3 million because they wouldn't get the payout that the teams that stay under the cap receive.  So in effect, it would cost the Wizards $6-7 million for someone who wouldn't have much of an impact while he's starting (because he's not familiar with the the team) and after a week or two he'd end up backing up Antonio Daniles (who only makes $5.8 million a year) before becoming a third stringer (fingers crossed) by February.  

Bottomline: The way that the salary cap is structured, it makes a whole lot more sense to go over the cap for a key player (like Arenas and Jamison, who are both free agents this year) then it does for a bench player.  It makes it tough for the next week or two, but in the long-term it makes a whole lot more sense, both in a strict financial sense and in not overpaying so-so players sense, to stay under right now.  Besides, the Wizards still have a higher payroll than 15 other teams, so it's not like Abe Pollin is the worst penny-pincher in the league, he'd just rather not spend money foolishly (like the Knicks).