If the spacing of the data is not completely regular, you could use griddata() or possibly interp2() to create a regular mesh grid.
If the spacing of the data is regular, then you can probably read the text file, extract the third column, and reshape() it (with possibly a transposition or two thrown in.)
[Edit: Moving part of answer from a comment to the answer - AU]
In the special case where your I and J are positive integers, you can use the following to build the matrix and fill any unused locations with the given FillValue:
FillValue = nan;
DataMatrix = accumarray(YourData(:,1:2),YourData(:,3),[],[],FillValue);
This will, though, construct the matrix indexed at 1 to max(I) and 1 to max(J); if your I and J span a subrange of that then it is more efficient to offset them when building the matrix. If your I or J can be non-positive then to use accumarray() you would have to offset them; e.g.,
Ioffset = min(YourData(:,1)) - 1;
Joffset = min(YourData(:,2)) - 1;
accumarray([YourData(:,1)-Ioffset, YourData(:,2)-Joffset], ...)
