The latest summary of national polls by Pollster.com suggests that he has.
If you look at Thompson (blue line) you see a rather dramatic decline over the last few weeks. Have Republicans been disappointed with his performance since his formal announcement?
Giuliani (in purple) has remained remarkably consistent. Romney (in Brown) and Huckabee (in Green) both seem to be mounting surges. Huckabee has farther to go and Romney has the money to back up his progress, but it seems both benefited from the departure of Brownback.
Ron Paul remains something of a fly in the ointment. Paul has grown his support to 2.2% but his positions are so unique that it is unlikely he can break into the Republican mainstream. It seems unlikely to me that Paul's anti-war supporters would willingly support another, pro-war Republican.
The Republican race is still fairly open. An argument could be made for any one of the top five pulling out a win in the Republican primary.