Since we've been attacking the dormant commerce clause (see here and here), I thought it would be good to quickly summarize the textual case against it. To be clear, there are enough historical arguments in its favor that I remain officially agnostic. But the textual case against it is powerful. Consider:
1. The commerce clause is a grant of power to Congress. On its face, it does not say anything to indicate it is an exclusive grant of power, or that it has any effect on the states.
2. The commerce clause appears in a section of the Constitution (Article I, Section 8) whose purpose is to grant powers to Congress. A number of the powers granted to Congress in this section, in terms indistinguishable from the commerce clause, are understood not to be exclusive powers. For example, the power to lay and collect taxes; the power to establish rules of naturalization and bankruptcies; the power over patents and copyrights; the power to punish piracies and felonies on the high seas (which states can do, at minimum, if the offender is a citizen of the state); the power to punish offenses against the law of nations (again, at least if the offender is a citizen of the state or the offense takes place in the state).
3. Also in Article I, Section 8, Congress is expressly declared to have "exclusive" power in one instance (the power "To exercise exclusive Legislation" over the capital district). Thus the absence of the word "exclusive" in the commerce clause is especially significant.
4. A different section -- Article I, Section 10 -- lists the things states cannot do. (E.g., states cannot enter into treaties, grant letters of marque and reprisal, coin money, etc.). The same section contains another list of things states cannot do without the consent of Congress (e.g., engage in war, keep troops or ships of war). Article I, Section 10 does not include regulating interstate commerce as one of the things states cannot do. Expresio unius, etc.
5. Article I, Section 10 contains a narrower restriction on state interference with interstate commerce: states cannot impose duties on imports or exports. Presumably these duties would be invalid under the dormant commerce clause (and thus this provision would be superfluous) if there actually was a dormant commerce clause in the text.
6. Many of the things prohibited to states in Article I, Section 10 are also granted to Congress in Article I, Section 8 (e.g., granting letters of marque and reprisal, coining money, keeping troops and ships of war, engaging in war). If a grant of power in Article I, Section 8 implies an exclusive grant (as the dormant commerce clause holds), then all these provisions of Article I, Section 10 are superfluous. To take a devastating example, Article I, Section 10 indicates that giving Congress the power to grant letters of marque and reprisal in Article I, Section 8 did not, in itself, deny states the power to grant them. Rather, in order to make letters of marque and reprisal an exclusive power of Congress, the Constitution also needed to say in Article I, Section 10 that states could not grant them (as it did).
7. In any event, even if one thought the grant to Congress of power to regulate commerce among the states could be read as a grant of exclusive power to regulate commerce among the states, that still would not provide a textual foundation for the modern dormant commerce clause. No one today thinks that states cannot regulate any commerce among the states. Rather, the dormant commerce clause doctrine is that (a) states cannot discriminate in their regulations of interstate commerce; and (b) states cannot impose an undue burden on interstate commerce, as balanced against the legitimate regulatory goals of the state. This rule simply cannot be reached by a claim that Congress has exclusive power to regulate interstate commerce.
While this textual evidence may not be conclusive (depending on what other evidence exists), at minimum proponents of the dormant commerce clause doctrine start with much to overcome.
(As an aside, I agree with Mike Rappaport that the privileges and immunities clause of Article IV may prohibit at least some of the discriminatory behavior presently prohibited under the dormant commerce clause doctrine. The most problematic part of the dormant commerce clause doctrine is the claim that states cannot unduly burden interstate commerce even with regulations that do not discriminate.)