Your actual party affiliations

[muon2]

Democratic | 2010-

Haven't registered to vote in Illinois yet, but will be unaffiliated when I do get around to it.

You don't have a choice. Party registration is not involved in the registration process here.

That said, make sure you register before early October when the deadlines are. And also make sure to at least vote for a couple Democrats such as Mike Frerichs for Treasurer, even if you don't like Quinn.

I can't vote for any Democrats in good conscience, given that I don't have much of anything  in common with them from an ideological standpoint.

The foundations are the same, but whatever. You might as well not vote, as the deadline for getting on the ballot was the other day and I am pretty sure the only one's who got on were the Dems, Pubs, and Libertarians.

What do you mean? The Democratic Party is a liberal party and I am not a liberal, so it doesn't really make much sense for me to vote for them in any context. There's not a single Democrat I can think of that I would support for any office at this point.

Assuming I get registered to vote in time, I plan on making extensive use of write-ins, and, barring that, just submitting a spoilt ballot.

Nonpartisan voter registration is available in IL through Oct 7, both in person at various government offices and online. From Oct 8 through election day Nov 4 you can register and vote through the grace period process at certain election offices in your county and at your polling place on election day. Depending on your county and whether you vote early you may not have a paper ballot to work with, though there is a process for write-ins on electronic ballots.

[muon2]

Democrat, till about 1999
Unaffiliated, 1999 till about 2003
Republican, 2003 - 2004
Unaffiliated 2004 - 2008
Republican, a few months in 2008
Unaffiliated 2008 - 2012
Republican, a few months in 2012
Unaffiliated, 2012 - 2014
Democrat, April 2014 to present (I haven't gotten around to changing it back to unaffiliated with the county registrar, but when I do I'll be Unafilliated till an interesting primary comes along)

I'd be interested in the reasoning behind all these...

It looks like someone who wants to have an impact at the primary. For states that have partisan registration, one must be registered with the party in advance of the primary. Most people won't bother to change it until the next primary comes along and they want to vote in a party primary other than the one they most recently did. Parties use those registration lists to reach out to possible supporters, so unregistering can cut down on that type of call.

IL has open party registration which means that a voter can declare their party intent when they walk up to the primary voting place. The upside is that the voter need not declare an affilation until the day of the vote. The downside is that there is no process to unregister, so for the next two years the party will contact the voter from the primary voting lists. If someone doesn't vote in a primary then IL maintains the last known affiliation on the marked partisan voter lists.