Generally, something becomes an LLC expense when the LLC pays for it (see note below). This implies that you have an LLC bank account with funds.
However, the IRS does recognize "business startup costs", which are expenses that occur before the business is established.
From IRS Publication 535 Business Expenses, page 26:
Start-up costs are amounts paid or incurred for: (a) creating an active trade or business; or (b) investigating the creation or acquisition of an active trade or business. Start-up costs include amounts paid or incurred in connection with an existing activity engaged in for profit; and for the production of income in anticipation of the activity becoming an active trade or business.
If there is equipment you purchased with your personal funds, but would now like the LLC to own them, the easiest thing to do would be to sell those items to the LLC at their current fair market value. Write yourself an expense report and transfer the value of the equipment from your business bank account to your personal bank account. There are other ways of handling this, but this is how I would do it to keep things simple.
Note: Determining the exact time when something becomes a business expense depends on your accounting method. There are two common accounting methods: cash basis and accrual basis. In cash basis accounting, you record an event (income or expense) when cash exchanges hands. In accrual basis accounting, you record the event when work has been performed, regardless of when the payment was made/received.
Let's say you performed services for a client in 2011, but don't receive the payment from the client until 2012. Under the cash method, that income will be recorded in 2012. Under the accrual method, the income will be recorded in 2011. The same applies to expenses.
For more on accounting methods see IRS Publication 538 Accounting Periods and Methods.