WordNet (r) 3.0 (2006):

tied up
    adj 1: kept occupied or engaged; "she's tied up at the moment
           and can't see you"; "the phone was tied up for almost an
           hour"

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "tied up":
   absorbed in, at it, at work, bound, burdened with debt, busy,
   caught up in, deep in debt, embarrassed, employed, encumbered,
   engaged, enmeshed in, entangled in, far-gone, fettered,
   full of business, hampered, handcuffed, hard at it, hard at work,
   immersed in, implicated in, in bonds, in chains, in debt,
   in difficulties, in embarrassed circumstances, in harness, in hock,
   in irons, in the hole, in the red, indebted, involved, involved in,
   ironbound, manacled, mortgaged, occupied, on duty, on the go,
   on the hop, on the job, on the jump, on the move, on the run,
   plunged in debt, shackled, strapped, submerged in, tethered, tied,
   tied down, tied up in, trammeled, working, wrapped up in