Additionally, the pro-Walker recall petitioners will need to have a cushion of signature over the 540,208, as some signatures are bound to be tossed out. How many more do they need? That's an open question. In California, they turned in over 1.3 million signatures for the Davis recall. This article lists a 10-15% signature error rate in a Michigan recall. But there are examples of recalls with many more signatures being invalidated. In this Phoenix city council recall, over 4,000 signatures were handed in. The recall only needed 2,329. It didn't make it. The error rate was close to, if not over, 50%.
Wisconsin does not seem to have overly strict standards for signatures (unlike NY's infamous law, which at one point let pen color invalidate pages of signatures). So that should help the pro-recall cause.
As Wisconsin doesn't have an initiative or referendum, the state does not possess California's highly developed signature gathering industry. On a state-wide basis, it's hard to tell how much a negative that will be. Let's look at last year. The pro-Walker recall forces got 6 of a possible 8 Republican state senate recalls on the ballot in arguably non-favorable districts (i.e. they didn't elect Democrats in 2010). The recall petitioners should be able to run up the score in the districts that elect Democrats. Wisconsin's law actually should provide some more comforting news for the Democrats. The senate recalls required the signatures of 25% of the votes cast for governor in that district in the last election. In other jurisdictions, it is a percentage of the votes cast for that particular office (which is invariably lower as people don't always vote the full ticket). This means that the Democrats were able to get the full support from needed for a Walker recall in all six of those Republican senate districts.
There's also the possibility of a "blocking" campaign or deliberately false signatures. This site, run by the people who claim to have gotten the Davis recall on the ballot, note that the Davis blocking campaign was extensive. Still, the blocking campaign failed. I don't think deliberately false signatures will have much of an effect -- the parties are keeping a running tally of whether the signatures look good. They will be able to weed out the obviously fake ones.
There's one big issue that augurs well for the Walker recall petitioners. It is money. There's going to be tons of following, much more than we saw in the Gray Davis recall. $44 million was spent on the nine Senate recalls. How much more is coming into a governor's race. And nothing helps a recall get on the ballot more than some ready cash.