Not likely magnesium deficiency then, it just looks like it (blotchy chlorosis
between the veins of the leaf).
I have not had spider mites, but there have been some recent threads
about them in the disease forum. If you click on the little button beside
the Search menu item at the top of the page, a little drop down menu
has Advanced Search. Click on that, you should be able to find the
spider mite threads with a keyword search (they do not necessarily
mention "spider mite" in the thread title).
Tomato disease identification sites:
http://vegetablemdonline.ppath.corne...omLeafKey.html
http://aggie-horticulture.tamu.edu/p...problemsolver/
http://www.omafra.gov.on.ca/IPM/engl...ly-blight.html
http://5e.plantphys.net/article.php?ch=5&id=289
(I usually start with the Cornell site, but in this case I think the Texas A&M
and Ontario sites have better Early Blight pictures.)